Rename .py to .pyi.

3/_warnings.pyi

@@ -0,0 +1,12 @@
+"""Stub file for the '_warnings' module."""
+# This is an autogenerated file. It serves as a starting point
+# for a more precise manual annotation of this module.
+# Feel free to edit the source below, but remove this header when you do.
+
+from typing import List, Tuple, Dict, Undefined, GenericType
+
+def _filters_mutated() -> None: ...
+
+def warn(message, *args, **kwargs) -> None: ...
+
+def warn_explicit(message, category, filename: str, lineno: int, *args, **kwargs) -> None: ...