Don’t version control Pytest cache
Don’t version control Pytest cache

Pytest moved its cache from `./.cache` (which is in our `.gitignore`) to
`./.pytest_cache` (which isn’t).

It’s annoying having to be careful not to commit it all the time, so
this commit makes it ignored.

See https://github.com/pytest-dev/pytest/issues/3286 for more context.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin