Use a set literal in _clean_settings (#1585)

AUTHORS

@@ -243,3 +243,4 @@ that much better:
  * Victor Varvaryuk
  * Stanislav Kaledin (https://github.com/sallyruthstruik)
  * Dmitry Yantsen (https://github.com/mrTable)
+ * Renjianxin (https://github.com/Davidrjx)

mongoengine/connection.py

@@ -146,13 +146,14 @@ def get_connection(alias=DEFAULT_CONNECTION_NAME, reconnect=False):
         raise MongoEngineConnectionError(msg)
 
     def _clean_settings(settings_dict):
-        irrelevant_fields = set([
+        # set literal more efficient than calling set function
-            'name', 'username', 'password', 'authentication_source',
+        irrelevant_fields_set = {
-            'authentication_mechanism'
+            'name', 'username', 'password',
-        ])
+            'authentication_source', 'authentication_mechanism'
+        }
         return {
             k: v for k, v in settings_dict.items()
-            if k not in irrelevant_fields
+            if k not in irrelevant_fields_set
         }
 
     # Retrieve a copy of the connection settings associated with the requested