python: Add flake8 to pipenv

python: Add flake8 to pipenv

flake8 3.5.x does not support the --extend-ignore syntax used in the
.flake8 file to gracefully extend default ignores, so 3.6.x is our
minimum requirement. There is no known upper bound.

flake8 can be run from the python/ directory with no arguments.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-21-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>