PYTHON-4780 Implement fast path for server selection with Primary

Author: blink1073

pymongo/topology_description.py

@@ -34,7 +34,7 @@
 from bson.objectid import ObjectId
 from pymongo import common
 from pymongo.errors import ConfigurationError, PyMongoError
-from pymongo.read_preferences import ReadPreference, _AggWritePref, _ServerMode
+from pymongo.read_preferences import Primary, ReadPreference, _AggWritePref, _ServerMode
 from pymongo.server_description import ServerDescription
 from pymongo.server_selectors import Selection
 from pymongo.server_type import SERVER_TYPE
@@ -326,7 +326,7 @@ def apply_selector(
 
         # Primary selection fast path.
         if self.topology_type == TOPOLOGY_TYPE.ReplicaSetWithPrimary and isinstance(
-            selector, TOPOLOGY_TYPE.Primary
+            selector, Primary
         ):
             for sd in self._server_descriptions.values():
                 if sd.server_type == SERVER_TYPE.RSPrimary: