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/util/retryutils.py

@@ -110,7 +110,7 @@ async def get_retry_limiter(destination, clock, store, ignore_backoff=False, **k
         failure_ts,
         retry_interval,
         backoff_on_failure=backoff_on_failure,
-        **kwargs
+        **kwargs,
     )