Add the flake8-import-order library to our test requirements so that we
can avoid these PEP8 violations and maintain consistent import ordering.
Also fix our violations.
This library is in requirements but is blacklisted from being
automatically updated since it is never shipped. For now, don't bother
to pin it.
Change-Id: I4e788292b98b7f2f835cc80081763b2d249fe43e