Add flake8 2.2.3

Flake8 2.2.2 didn't actually turn concurrency on by default, flake8
2.2.3 does.

The move to flake8 2.2.3 is done in two steps since our
global-requirements system does now allow for non-overlapping changes.

Bug: https://bitbucket.org/tarek/flake8/issue/163/
Change-Id: I676dbf2402922112923d63a98b1aae85abe0331a
Merge "Add flake8 2.2.3"