Sphinx is Python-3-only and therefore is not a universal wheel. It
should still distribute a 'py3' wheel, just not a universal one.

From https://wheel.readthedocs.io/en/stable/user_guide.html

> If your project contains no C extensions and is expected to work on
> both Python 2 and 3, you will want to tell wheel to produce universal
> wheels by adding this to your setup.cfg file: …

As Sphinx doesn't work on Python 2, it should not be universal.

Refs #6470

To stop PytestUnknownMarkWarning, this declares markers on setup.cfg
https://docs.pytest.org/en/latest/mark.html

The mypy config 'incremental` has defaulted as true since version 0.590.
Can drop the local override. For details, see:

https://github.com/python/mypy/commit/6b13652a466ccb102987a2ab1fa93d4b52fd8e3f

Since pytest-3.9, precedence order of warning options had been
changed.  This moves ``-W all`` option into setup.cfg.

In detail, see https://github.com/pytest-dev/pytest/issues/3946

Signed-off-by: Stephen Finucane <stephen@that.guru>

Since mypy-0.600, validations for Optional becomes very strict. It
interfere our development. So this disables it temporarily.