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/pwd.pyi

@@ -0,0 +1,14 @@
+from typing import List, Tuple
+
+class struct_passwd(Tuple[str, str, int, int, str, str, str]):
+    pw_name: str
+    pw_passwd: str
+    pw_uid: int
+    pw_gid: int
+    pw_gecos: str
+    pw_dir: str
+    pw_shell: str
+
+def getpwall() -> List[struct_passwd]: ...
+def getpwuid(__uid: int) -> struct_passwd: ...
+def getpwnam(__name: str) -> struct_passwd: ...