Does not work with pytest-xdist

[iurisilvio]

Running with xdist, each worker and the controller execute the same code.

In the case of the controller, cache._flake8mtimes is never filled. Each worker generate a cache for part of the collected files.

At the end of execution, each worker write their data to the same cache (overwriting previous writes) and, after that, the controller write to cache an empty cache.

pytest-flake8/pytest_flake8.py

Line 84 in d730278

config.cache.set(HISTKEY, config._flake8mtimes)