iotests/297: Have mypy ignore unused ignores

iotests/297: Have mypy ignore unused ignores

e7874a50ff3f5b20fb46f36958ad ("python: update for mypy 0.950") has added
`warn_unused_ignores = False` to python/setup.cfg, to be able to keep
compatibility with both pre- and post-0.950 mypy versions.

The iotests' mypy.ini needs the same, or 297 will fail (on both pre- and
post-0.950 mypy, as far as I can tell; just for different `ignore`
lines).

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220621092536.19837-1-hreitz@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>