Convert internal pusher dicts to attrs classes. (#8940)

This improves type hinting and should use less memory.

synapse/rest/admin/users.py

@@ -42,17 +42,6 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
-_GET_PUSHERS_ALLOWED_KEYS = {
-    "app_display_name",
-    "app_id",
-    "data",
-    "device_display_name",
-    "kind",
-    "lang",
-    "profile_tag",
-    "pushkey",
-}
-
 
 class UsersRestServlet(RestServlet):
     PATTERNS = admin_patterns("/users/(?P<user_id>[^/]*)$")
@@ -770,10 +759,7 @@ class PushersRestServlet(RestServlet):
 
         pushers = await self.store.get_pushers_by_user_id(user_id)
 
-        filtered_pushers = [
-            {k: v for k, v in p.items() if k in _GET_PUSHERS_ALLOWED_KEYS}
-            for p in pushers
-        ]
+        filtered_pushers = [p.as_dict() for p in pushers]
 
         return 200, {"pushers": filtered_pushers, "total": len(filtered_pushers)}