Upgrade to Python 3.9 and add some of the newer Python magic we have

 * 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

.python-version

-3.6.9
+3.9.4

Makefile

@@ -35,11 +35,11 @@ lint: node_modules/.uptodate
 
 .PHONY: docs
 docs: python
-	tox -e py36-docs
+	tox -e docs
 
 .PHONY: checkdocs
 checkdocs: python
-	tox -e py36-checkdocs
+	tox -e checkdocs
 
 .PHONY: clean
 clean:

requirements/docs.in

+recommonmark
+sphinx-autobuild
+sphinx
+sphinx_rtd_theme
\ No newline at end of file

requirements/docs.txt

+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile requirements/docs.in
+#
+alabaster==0.7.12
+    # via sphinx
+babel==2.9.1
+    # via sphinx
+certifi==2020.12.5
+    # via requests
+chardet==4.0.0
+    # via requests
+colorama==0.4.4
+    # via sphinx-autobuild
+commonmark==0.9.1
+    # via recommonmark
+docutils==0.16
+    # via
+    #   recommonmark
+    #   sphinx
+    #   sphinx-rtd-theme
+idna==2.10
+    # via requests
+imagesize==1.2.0
+    # via sphinx
+jinja2==2.11.3
+    # via sphinx
+livereload==2.6.3
+    # via sphinx-autobuild
+markupsafe==1.1.1
+    # via jinja2
+packaging==20.9
+    # via sphinx
+pygments==2.9.0
+    # via sphinx
+pyparsing==2.4.7
+    # via packaging
+pytz==2021.1
+    # via babel
+recommonmark==0.7.1
+    # via -r requirements/docs.in
+requests==2.25.1
+    # via sphinx
+six==1.16.0
+    # via livereload
+snowballstemmer==2.1.0
+    # via sphinx
+sphinx-autobuild==2021.3.14
+    # via -r requirements/docs.in
+sphinx-rtd-theme==0.5.2
+    # via -r requirements/docs.in
+sphinx==3.5.4
+    # via
+    #   -r requirements/docs.in
+    #   recommonmark
+    #   sphinx-autobuild
+    #   sphinx-rtd-theme
+sphinxcontrib-applehelp==1.0.2
+    # via sphinx
+sphinxcontrib-devhelp==1.0.2
+    # via sphinx
+sphinxcontrib-htmlhelp==1.0.3
+    # via sphinx
+sphinxcontrib-jsmath==1.0.1
+    # via sphinx
+sphinxcontrib-qthelp==1.0.3
+    # via sphinx
+sphinxcontrib-serializinghtml==1.1.4
+    # via sphinx
+tornado==6.1
+    # via livereload
+urllib3==1.26.4
+    # via requests
+
+# The following packages are considered to be unsafe in a requirements file:
+# setuptools

tox.ini

 [tox]
-envlist = py3-docs
+envlist = docs
 skipsdist = true
+requires =
+  tox-pip-sync
+  tox-pyenv
+  tox-run-command
 
 [testenv]
 skip_install = true
 deps =
-    {docs,checkdocs}: recommonmark
-    {docs,checkdocs}: sphinx-autobuild
-    {docs,checkdocs}: sphinx
-    {docs,checkdocs}: sphinx_rtd_theme
+    {docs,checkdocs}: -r requirements/docs.txt
+    {env:EXTRA_DEPS:}
 passenv =
     HOME
-changedir =
-    {docs,checkdocs}: docs
+    EXTRA_DEPS
 commands =
-    docs: sphinx-autobuild -qT --open-browser -b dirhtml -d {envdir}/doctrees . {envdir}/html
-    checkdocs: sphinx-build -qTWn -b dirhtml -d {envdir}/doctrees . {envdir}/html
+    docs: sphinx-autobuild -qT --open-browser -b dirhtml -d {envdir}/doctrees docs {envdir}/html
+    checkdocs: sphinx-build -qTWn -b dirhtml -d {envdir}/doctrees docs {envdir}/html