Rename .py to .pyi.

3/atexit.pyi

@@ -0,0 +1,18 @@
+"""Stub file for the 'atexit' 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 _clear() -> None: ...
+
+def _ncallbacks() -> long: ...
+
+def _run_exitfuncs() -> None: ...
+
+def register(*args, **kwargs) -> object:
+    raise MemoryError()
+    raise TypeError()
+
+def unregister(*args, **kwargs) -> None: ...