* Refactor the documentation build process
Significant improvements:
* Allow easy isolated local builds via a dedicated Tox environment
* Configure Sphinx to use consistent packages and theme across local, CI,
and ReadTheDocs builds; local builds now look the same as the published
RTD pages
* Explicitly add a ReadTheDocs configuration as code, per their documented
best practices
* Remove lots of dead code and simplify the Sphinx configuration file
* Build docs in a dedicated CI step and enable stricter warning checking
* Fix docutils related build error.
* Relax Sphinx version.
* Invalidate RTD build cache.
* No need to combine coverage reporting I think.
* Add combining again.
Co-authored-by: Jannis Leidel <jannis@leidel.info>
Without this, using `configurations.setup()` after Django has been setup
already re-triggers the logging configuration, which then causes e.g.
pytest's caplog to not work anymore.
- Drop django==1.9 as it reached end of life
- Drop python2.6, python3.3, and bring python3.6
- Bring django-2.0 in the matrix as expected failure until we add its
support
