Tell Black to format code for Python 3.5 (#8664)

This allows trailing commas in multi-line arg lists.

Minor, but we might as well keep our formatting current with regard to
our minimum supported Python version.

Signed-off-by: Dan Callahan <danc@element.io>

tests/test_utils/event_injection.py

@@ -50,7 +50,7 @@ async def inject_member_event(
         sender=sender,
         state_key=target,
         content=content,
-        **kwargs
+        **kwargs,
     )