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/tabnanny.pyi

@@ -0,0 +1,14 @@
+from _typeshed import AnyPath
+from typing import Iterable, Tuple
+
+verbose: int
+filename_only: int
+
+class NannyNag(Exception):
+    def __init__(self, lineno: int, msg: str, line: str) -> None: ...
+    def get_lineno(self) -> int: ...
+    def get_msg(self) -> str: ...
+    def get_line(self) -> str: ...
+
+def check(file: AnyPath) -> None: ...
+def process_tokens(tokens: Iterable[Tuple[int, str, Tuple[int, int], Tuple[int, int], str]]) -> None: ...