New function `repr_args` in `pprint` or `reprlib`

[blhsing (Ben Hsing)]

@Lucas_Malor

In this way the working code will be under unittest.mock. Quite confusing.
IMO the new code should use reprlib, and old code should adopt reprlib, if there’s someone so willing to do the cleaning work.

No, what I suggested is that we can move the current private function to reprlib, and have unittest.mock import the private function from reprlib instead.