[tests] Require flake8 release 3.6.0

flake8 tests fails with OptionConflictError for release 3.7.0-3.7.2
because flake8-per-file-ignores is part of flake8 now.

This problem has been solved with 3.7.3 but doctests often needs 15 minutes
and more and test will be aborted.

Therefore use flake8 release 3.6.0 further.

Also fix some pep issues found by flake8 3.7

Bug: T214962
Change-Id: I00f7ed97d73344480f12194de6b102b70f0d6d48