Add doc8 check to docs builds
Currently, doc8 only runs in the pep8 testenv.  Add it to the docs
testenv to make it easier not to submit a patch with an error.
@@ -123,8 +123,10 @@ commands = {posargs}
[testenv:docs]
allowlist_externals = rm
deps =
+ doc8
-r{toxinidir}/doc/requirements.txt
commands =
+ doc8
rm -rf doc/source/contributor/api doc/build/html doc/build/doctrees
sphinx-build -W -j auto -b html -d doc/build/doctrees doc/source doc/build/html
# Test the redirects. This must run after the main docs build
