[PyMySQL] Add Cursor.warning_count attribute (#15150)

Author: scottnemes

stubs/PyMySQL/pymysql/cursors.pyi

@@ -17,6 +17,7 @@ class Cursor:
     messages: Any
     errorhandler: Any
     lastrowid: int
+    warning_count: int
     def __init__(self, connection: Connection[Any]) -> None: ...
     def close(self) -> None: ...
     def setinputsizes(self, *args) -> None: ...