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/nis.pyi

@@ -0,0 +1,9 @@
+import sys
+from typing import Dict, List
+
+if sys.platform != "win32":
+    def cat(map: str, domain: str = ...) -> Dict[str, str]: ...
+    def get_default_domain() -> str: ...
+    def maps(domain: str = ...) -> List[str]: ...
+    def match(key: str, map: str, domain: str = ...) -> str: ...
+    class error(Exception): ...