iotests: split linters.py out from 297

iotests: split linters.py out from 297

Now, 297 is just the iotests-specific incantations and linters.py is as
minimal as I can think to make it. The only remaining element in here
that ought to be configuration and not code is the list of skip files,
but they're still numerous enough that repeating them for mypy and
pylint configurations both would be ... a hassle.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-id: 20211019144918.3159078-12-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>