Switch the markdown parser to the one recommended by the current Sphinx
documentation [1].

This fixes an error parsing Markdown code blocks in `adr-003.md` when
running `make checkdocs` and makes this file show up under "Architecture
Decision Records" in the generated docs.

The `requirements/docs.txt` file was updated by directly running
`.tox/docs/bin/pip-compile -r requirements/docs.in`.

[1] https://www.sphinx-doc.org/en/master/usage/markdown.html

 * This adds pinned versions of the docs dependencies, so they don't
 drift between local and CI
 * This is managed with `hdev requirements` so support for that has
 been added
 * Using the pyenv integration we have elsewhere so tox tracks the
 versions in the version file

9bec434d moved the logic for scrolling
to an anchor into the document integrations, in order to enable
custom logic depending on the document type. The bucket bar however
was still using a generic approach.

Change the bucket bar to use the same `scrollToAnchor` implementation as
when scrolling is triggered by clicking an annotation card in the
sidebar.

 - Add `scrollToAnchor` method to `Guest`
 - Modify bucket bar controller to use this method

Related to https://github.com/hypothesis/client/issues/3269

 - Rename `reanchoringWait` to `reanchoringMaxWait` to make it clear
   that this is the maximum time that we'll wait
 - Rename {initialOffset, targetOffset} => {startOffset, endOffset} for
   clarity
 - Add comment to explain a sleep

The scenario this function tests for is an anchor's highlight being
_inside_ a placeholder element rather than _being_ a placeholder
element.

When scrolling to an anchor in a page that has not been rendered by
PDF.js, the anchor actually references a placeholder element in the
middle of the page. In order to scroll to the correct location in the
page scrolling for such anchors needs to happen in three phases:

 1. Scroll to the approximate location of the final anchor, given by the
    placeholder anchor. This will trigger PDF.js to re-render the target
    page.
 2. Wait for PDF.js to finish re-rendering the page and for the client
    to finish re-anchoring the annotation.
 3. Scroll to the real/non-placeholder anchor.

Change `PDFIntegration#scrollToAnchor` to implement the above steps.

Fixes https://github.com/hypothesis/client/issues/3269

This uses the real `Intl.DateTimeFormat` API but with a fixed locale to
ensure consistent output across systems. This is a more complete test
than the previous one.

The test assumes that output is consistent across browsers for the date
given. In testing with Safari, Firefox and Chrome this was the case for
the en-US locale. We accept some small risk that the exact format might change
slightly in future releases of one of these browsers.

Previously `decayingInterval` exhibited reasonable behavior if the date
was invalid by never invoking the callback. However this was not easy to
see from the code and not tested. Add explicit checks in `nextFuzzyUpdate` to
make this more obvious.

 - Combine `sidebar/util/date.js` and `sidebar/util/time.js`. It was
   confusing to have two separate modules when there wasn't a clear
   separation of "time utilities" vs "date utilities". This also
   eliminates some redundant caching logic.

 - Rename `toFuzzyString` and `format` to `formatRelativeDate` and
   `formatDate` respectively to make the differnce between them more
   obvious

 - Add missing test for `formatDate`

 - Remove the fallback code paths for old browsers that don't support
   `Intl.DateTimeFormat`. All of our currently supported browsers do.

 - Remove unused `date` parameter from `decayingInterval` callback. The
   only consumer in `AnnotationTimestamps` didn't use it.

 - Improve types in `time.js`

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.12 to 8.2.13.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.2.12...8.2.13)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [katex](https://github.com/KaTeX/KaTeX) from 0.13.3 to 0.13.5.
- [Release notes](https://github.com/KaTeX/KaTeX/releases)
- [Changelog](https://github.com/KaTeX/KaTeX/blob/master/CHANGELOG.md)
- [Commits](https://github.com/KaTeX/KaTeX/compare/v0.13.3...v0.13.5)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 6.3.1 to 6.3.5.
- [Release notes](https://github.com/getsentry/sentry-javascript/releases)
- [Changelog](https://github.com/getsentry/sentry-javascript/blob/master/CHANGELOG.md)
- [Commits](https://github.com/getsentry/sentry-javascript/compare/6.3.1...6.3.5)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.13.15 to 7.14.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.14.0/packages/babel-preset-env)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [gulp-replace](https://github.com/lazd/gulp-replace) from 1.1.1 to 1.1.2.
- [Release notes](https://github.com/lazd/gulp-replace/releases)
- [Commits](https://github.com/lazd/gulp-replace/compare/v1.1.1...v1.1.2)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [core-js](https://github.com/zloirock/core-js/tree/HEAD/packages/core-js) from 3.11.0 to 3.11.2.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/commits/v3.11.2/packages/core-js)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.13.16 to 7.14.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.14.0/packages/babel-core)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [sass](https://github.com/sass/dart-sass) from 1.32.11 to 1.32.12.
- [Release notes](https://github.com/sass/dart-sass/releases)
- [Changelog](https://github.com/sass/dart-sass/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sass/dart-sass/compare/1.32.11...1.32.12)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [axe-core](https://github.com/dequelabs/axe-core) from 4.1.4 to 4.2.0.
- [Release notes](https://github.com/dequelabs/axe-core/releases)
- [Changelog](https://github.com/dequelabs/axe-core/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/dequelabs/axe-core/compare/v4.1.4...v4.2.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.892.0 to 2.897.0.
- [Release notes](https://github.com/aws/aws-sdk-js/releases)
- [Changelog](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/aws/aws-sdk-js/compare/v2.892.0...v2.897.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [dompurify](https://github.com/cure53/DOMPurify) from 2.2.7 to 2.2.8.
- [Release notes](https://github.com/cure53/DOMPurify/releases)
- [Commits](https://github.com/cure53/DOMPurify/compare/2.2.7...2.2.8)
Signed-off-by: dependabot[bot] <support@github.com>

`uorigin` is un-idiomatic naming in our codebase, though the meaning was
fairly obvious in context.

Modernize the `RPC` class in `src/shared/frame-rpc` that serves as both
the client and server for RPC requests between frames.

This code was originally imported from
https://github.com/substack/frame-rpc but now that we've adopted it, it
makes sense to modernize it to match the conventions and requirements of
the rest of our code.

 - Convert `frame-rpc.js` to modern syntax
 - Update JSDoc documentation for methods and class
 - Convert `RPC` class to a named rather than default export
 - Inline the `apply` method into the `call` method, since only `call`
   was used externally
 - Remove unused option for `methods` constructor argument to be a
   function
 - Rename `src` and `dest` public fields to the more explicit
   `sourceFrame` and `destFrame`

Convert this service to a class and expand the documentation slightly.
This is a straightforward conversion with no API or internal changes.

Part of https://github.com/hypothesis/client/issues/3298

The current error handling is not that useful in practice because it
just results in a console error. With a test in place we can more easily
change/refactor this in future though.

 - Document parameters and return types
 - Add a link to the documentation for the available options. We
   currently only ever use `minTimeout`
 - Add a note about why the code calls `operation.retry()` even if the
   operation was successful

 - Convert closures into methods

 - Remove the `profileFetchRetryOpts` field that was used only to shorten the
   delay between retries in tests, in favor of mocking
   `retryPromiseOperation` in tests to remove the delay between retries.

 - Change `SessionService` instance creation in tests to allow
   individual tests to run custom setup logic before the service is
   constructed. This was needed due to allow the `serviceConfig` mock to
   take effect when the `SessionService` constructor runs

 - Remove unnecessary custom Sinon sandbox in tests

This is a straightforward conversion, except for the removal of an
unused `state` property on the instance.

Part of https://github.com/hypothesis/client/issues/3298

First scenario tests the injection of the client on child iframe. The
child iframe is made annotatable, by having an
`enable-annotation="true"` attribute and sharing the same origin. The
injection of the annotator happens after the the sidebar is created,
thus testing if the child iframe is able to discover the sidebar iframe
and communicate with it via postMessages.

In the second scenario, the host page contains two children iframes,
each with its own client. Every annotator/sidebar combo on each frame
should work independently from each other and shouldn't pass any message
across frames.