Add a shared docs build env dependencies location

This change adds a new dependencies list location that is compatible
with:

  * pip-tools
  * dependabot
  * pip itself

It allows us to gradually introduce support for locked build envs
with pinned versions and hashes following the best practices present
in many major projects in Python ecosystem (like PyPI) while keeping
the DX with our main (tox-oriented) dev setup seamless.

Refs:

  * https://hynek.me/articles/python-app-deps-2018/#pip-tools--everything-old-is-new-again
  * https://github.com/jazzband/pip-tools#output-file
  * https://github.com/pypa/warehouse/tree/master/requirements
  * https://dependabot.com/blog/dependabot-now-supports-pip-compile/
  * dependabot/feedback#658 (comment)
  * https://github.com/dependabot/dependabot-core/blob/20557d2/python/lib/dependabot/python/update_checker/pip_compile_version_resolver.rb#L23

.readthedocs.yml

@@ -35,5 +35,6 @@ build:
 # to build docs
 python:
   version: 3.8
-  install: []
+  install:
+  - requirements: docs/requirements.in
   system_packages: false

docs/requirements.in

@@ -0,0 +1 @@
+Sphinx

tox.ini

@@ -104,7 +104,8 @@ passenv =
 
 [testenv:docs]
 basepython = python3
-deps = sphinx
+deps =
+  -r{toxinidir}/docs/requirements.in
 commands =
   # Generate an RST rules table from the rules in Python modules:
   sh -c "{envpython} -m ansiblelint -L -f rst > rst/rules/default_rules.rst"