action/append should `push` new values, not `concat`


In `test_argparse.py` `TestPositionalsActionAppend` has 2 positional args, one that takes 1 argument, the other 2.
`parseArgs` with `'a b c'` produces `spam: ['a','b','c']`, but python expects `spam: ['a', ['b','c']]`.
This is because append.js uses `concat`, while python uses `append`.

Correction: change `action/append.js set()` to use `push()`