* commit '837293c31':
Remove obsolete __future__ imports (#8337)
Use admin_patterns for all admin APIs. (#8331)
Fix a potential bug of UnboundLocalError (#8329)
Switch metaclass initialization to python 3-compatible syntax (#8326)
Catch-up after Federation Outage (split, 4): catch-up loop (#8272)
Use slots in attrs classes where possible (#8296)
Fix typos in comments.
Add the topic and avatar to the room details admin API (#8305)
Improve SAML error messages (#8248)
Add experimental support for sharding event persister. Again. (#8294)
Make `StreamToken.room_key` be a `RoomStreamToken` instance. (#8281)
Use TLSv1.2 for fake servers in tests (#8208)
Add /_synapse/client to the reverse proxy docs (#8227)
Clean up `Notifier.on_new_room_event` code path (#8288)