Use patch.object instead of assignments

Signed-off-by: Quentin Gliech <quenting@element.io>

tests/unittest.py

@@ -321,10 +321,12 @@ async def get_requester(*args, **kwargs) -> Requester:
                         access_token_id=token_id,
                     )
 
-                # Type ignore: mypy doesn't like us assigning to methods.
-                self.hs.get_auth().get_user_by_req = get_requester  # type: ignore[assignment]
-                self.hs.get_auth().get_user_by_access_token = get_requester  # type: ignore[assignment]
-                self.hs.get_auth().get_access_token_from_request = Mock(return_value=token)  # type: ignore[assignment]
+                get_token = Mock(return_value=token)
+
+                auth = self.hs.get_auth()
+                patch.object(auth, "get_user_by_req", new=get_requester)
+                patch.object(auth, "get_user_by_access_token", new=get_requester)
+                patch.object(auth, "get_access_token_from_request", new=get_token)
 
         if self.needs_threadpool:
             self.reactor.threadpool = ThreadPool()  # type: ignore[assignment]