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>

synapse/http/client.py

@@ -359,7 +359,7 @@ class SimpleHttpClient:
                     agent=self.agent,
                     data=body_producer,
                     headers=headers,
-                    **self._extra_treq_args
+                    **self._extra_treq_args,
                 )  # type: defer.Deferred
 
                 # we use our own timeout mechanism rather than treq's as a workaround