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/marshal.pyi

@@ -0,0 +1,8 @@
+from typing import IO, Any
+
+version: int
+
+def dump(__value: Any, __file: IO[Any], __version: int = ...) -> None: ...
+def load(__file: IO[Any]) -> Any: ...
+def dumps(__value: Any, __version: int = ...) -> bytes: ...
+def loads(__bytes: bytes) -> Any: ...