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/rlcompleter.pyi

@@ -0,0 +1,11 @@
+import sys
+from typing import Any, Dict, Optional, Union
+
+if sys.version_info >= (3,):
+    _Text = str
+else:
+    _Text = Union[str, unicode]
+
+class Completer:
+    def __init__(self, namespace: Optional[Dict[str, Any]] = ...) -> None: ...
+    def complete(self, text: _Text, state: int) -> Optional[str]: ...