Fix flake8 warnings in plugins.py

There is some inconsistency between the formatting done by black
and the format expected by 'flake8'. Add flake8 suppressions for
those.

Also configure flake8 to allow maximum line length of 100 columns
rather than the standard 80 columns.

Change-Id: Id40715fd7a528d3743b67bff7eb2138bc41df07a
2 files changed