qapi: enable strict-optional checks

In the modules that we are checking so far, we can be stricter about the
difference between Optional[T] and T types. Enable that check.

Enabling it now will assist review on further typing and cleanup work.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210201193747.2169670-17-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/mypy.ini

@@ -1,6 +1,5 @@
 [mypy]
 strict = True
-strict_optional = False
 disallow_untyped_calls = False
 python_version = 3.6