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/replication/_base.py

@@ -269,7 +269,7 @@ class BaseMultiWorkerStreamTestCase(unittest.HomeserverTestCase):
             homeserver_to_use=GenericWorkerServer,
             config=config,
             reactor=self.reactor,
-            **kwargs
+            **kwargs,
         )
 
         # If the instance is in the `instance_map` config then workers may try