mailmap: Fix mypy errors with django-stubs 5.0.2
Fix following errors:
swh/web/mailmap/management/commands/sync_mailmaps.py:49: error: "QuerySet" expects 2 type arguments, but 1 given [type-arg]
swh/web/mailmap/management/commands/sync_mailmaps.py:62: error: "QuerySet" expects 2 type arguments, but 1 given [type-arg]