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/grp.pyi

@@ -0,0 +1,11 @@
+from typing import List, NamedTuple, Optional
+
+class struct_group(NamedTuple):
+    gr_name: str
+    gr_passwd: Optional[str]
+    gr_gid: int
+    gr_mem: List[str]
+
+def getgrall() -> List[struct_group]: ...
+def getgrgid(id: int) -> struct_group: ...
+def getgrnam(name: str) -> struct_group: ...