qapi/schema: turn on mypy strictness

This patch can be rolled in with the previous one once the series is
ready for merge, but for work-in-progress' sake, it's separate here.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240315152301.3621858-23-armbru@redhat.com>

scripts/qapi/mypy.ini

@@ -2,8 +2,3 @@
 strict = True
 disallow_untyped_calls = False
 python_version = 3.8
-
-[mypy-qapi.schema]
-disallow_untyped_defs = False
-disallow_incomplete_defs = False
-check_untyped_defs = False