Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/macurl2path.pyi

@@ -0,0 +1,5 @@
+from typing import Union
+
+def url2pathname(pathname: str) -> str: ...
+def pathname2url(pathname: str) -> str: ...
+def _pncomp2url(component: Union[str, bytes]) -> str: ...