PYTHON-3065 Ignore SRV polling update when topology is discovered to be a replica set (#943)

(cherry picked from commit cbab615)

Author: ShaneHarvey

pymongo/topology.py

@@ -340,6 +340,8 @@ def _process_srv_update(self, seedlist):
         Hold the lock when calling this.
         """
         td_old = self._description
+        if td_old.topology_type not in SRV_POLLING_TOPOLOGIES:
+            return
         self._description = _updated_topology_description_srv_polling(self._description, seedlist)
 
         self._update_servers()

pymongo/topology_description.py

@@ -446,6 +446,7 @@ def _updated_topology_description_srv_polling(topology_description, seedlist):
       - `seedlist`: a list of new seeds new ServerDescription that resulted from
         a hello call
     """
+    assert topology_description.topology_type in SRV_POLLING_TOPOLOGIES
     # Create a copy of the server descriptions.
     sds = topology_description.server_descriptions()