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/mailcap.pyi

@@ -0,0 +1,8 @@
+from typing import Dict, List, Mapping, Optional, Sequence, Tuple, Union
+
+_Cap = Dict[str, Union[str, int]]
+
+def findmatch(
+    caps: Mapping[str, List[_Cap]], MIMEtype: str, key: str = ..., filename: str = ..., plist: Sequence[str] = ...
+) -> Tuple[Optional[str], Optional[_Cap]]: ...
+def getcaps() -> Dict[str, List[_Cap]]: ...