Add flake8 2.2.4

Flake8 2.2.4 Fix bugs triggered by turning multiprocessing on by default.

The move to flake8 2.2.4 is done in two steps since our
global-requirements system does not allow non-overlapping changes.

Change-Id: Ifb1c91ccfac7dd71ef0641f3a8660b4e492ecfa4
Merge "Add flake8 2.2.4"