python: add excluded dirs to flake8 config

python: add excluded dirs to flake8 config

Instruct flake8 to avoid certain well-known directories created by
python tooling that it ought not check.

Note that at-present, nothing actually creates a ".venv" directory; but
it is in such widespread usage as a de-facto location for a developer's
virtual environment that it should be excluded anyway. A forthcoming
commit canonizes this with a "make venv" command.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-20-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>