Add new Pytest cache directory to gitignore

Starting with Pytest 3.4.0 (2018-01-30), Pytest's cache directory was
renamed to .pytest_cache.

https://docs.pytest.org/en/latest/changelog.html#pytest-3-4-0-2018-01-30

> The default cache directory has been renamed from .cache to
> .pytest_cache after community feedback that the name .cache did not
> make it clear that it was used by pytest.