The new version enables a lot of standard flake8 checks. Some of them
are temporary disabled to reduce the scope of this patch:
* Complexity check requires a few functions to be rewritten (apparently,
it was not enabled previously).
* Indentation check failures are numerous and potentially contradictive.
These checks will be enabled in follow-ups.
W606 is removed from excludes since we no longer hit it.
Change-Id: I1e5a6f8e5e90c55cfc6f740b26c30196512d3be3
a3d7d73a69