Update black, and run auto formatting over the codebase (#9381)

- Update black version to the latest
 - Run black auto formatting over the codebase
    - Run autoformatting according to [`docs/code_style.md
`](80d6dc9783/docs/code_style.md)
 - Update `code_style.md` docs around installing black to use the correct version

synapse/events/builder.py

@@ -98,7 +98,9 @@ class EventBuilder:
         return self._state_key is not None
 
     async def build(
-        self, prev_event_ids: List[str], auth_event_ids: Optional[List[str]],
+        self,
+        prev_event_ids: List[str],
+        auth_event_ids: Optional[List[str]],
     ) -> EventBase:
         """Transform into a fully signed and hashed event