pymysql.err.Warning multiple-inherits from builtins.Warning and MySQLError. (#4303)

Previously we declared it as single-inheriting from only `MySQLError`.

third_party/2and3/pymysql/err.pyi

@@ -1,9 +1,10 @@
+import builtins
 from typing import Dict, NoReturn, Type
 
 from .constants import ER as ER
 
 class MySQLError(Exception): ...
-class Warning(MySQLError): ...
+class Warning(builtins.Warning, MySQLError): ...
 class Error(MySQLError): ...
 class InterfaceError(Error): ...
 class DatabaseError(Error): ...