pytest.ini is not found if running the test suite from anywhere but
a source checkout root.  For example, it is not found if running
`matplotlib.test()` from an installed Matplotlib (that includes test
data). mplcairo's test suite also relies on being able to run the
Matplotlib test suite from not-the-source-tree (via `pytest --pyargs
matplotlib`).

This is particularly problematic now that pytest requires markers to be
explicitly declared, and emits a warning when they are not.

Instead, declare most relevant options in pytest_configure().  (Note
that this actually only helps mplcairo when used in conjunction of
pytest>=5.0 due to pytest issue 5078, but the patch doesn't bump the
pytest version requirement for matplotlib.)

See also pytest issue 4039.