Fix pytest 3.4.0 cache gitignore

[OttoWinter]

Description:

In #13169 pytest was upgraded to 3.4.2, but with that upgrade pytest also changed the pytest cache directory to .pytest_cache: https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst

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.

Missed that when scrolling through the logs, sorry about that!

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass