iotests/linters: check mypy files all at once

We can circumvent the '__main__' redefinition problem by passing
--scripts-are-modules. Take mypy out of the loop per-filename and check
everything in one go: it's quite a bit faster.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210923180715.4168522-4-jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>