add (overwrite with) mypy stubs, if available

builtins/3/grp.pyi

@@ -0,0 +1,13 @@
+from typing import List
+
+# TODO group database entry object type
+
+class struct_group:
+    gr_name = ''
+    gr_passwd = ''
+    gr_gid = 0
+    gr_mem = ...  # type: List[str]
+
+def getgrgid(gid: int) -> struct_group: ...
+def getgrnam(name: str) -> struct_group: ...
+def getgrall() -> List[struct_group]: ...