Switch to openstackdocstheme 2.2.1 and reno 3.1.0 versions. Using
these versions will allow especially:
* Linking from HTML to PDF document
* parallelizing building of documents

Update Sphinx version as well.

openstackdocstheme renames some variables, so follow the renames. A
couple of variables are also not needed anymore, remove them.

Set openstackdocs_pdf_link to link to PDF file.

Change pygments_style to 'native' since old theme version always used
'native' and the theme now respects the setting and using 'sphinx' can
lead to some strange rendering.

Depends-On: https://review.opendev.org/729744
Change-Id: I311a5daa382dfca07e618eb6cbb3f44bd0502b02

python_version==2.7 is not supported anymore, remove this line to
unbreak the repo.

Change-Id: I15de57f071a3080239a4418561e4ad610289e478

Python 3.8 is now our highest level supported python runtime.
This updates the default tox target environments to swap out
py37 for py38 to make sure local development testing is
covering this version.

This does not impact zuul jobs in any way, nor prevent local
tests against py37. It just changes the default if none is
explicitly provided.

Change-Id: I35b2404890339a4e1f18adf49a2de58d45bb1523
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

Now that we are running the Victoria tests that include a
voting py38, we can now add the Python 3.8 metadata to the
package information to reflect that support.

Change-Id: I49640c50a7b78212a5c2ad03acb43be7adf98305
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

Fix the typo for the attribute word in comment and docstring.

Change-Id: Ic4a841c333d712a22503c1d7dc2d21619c601408