Convert more functions in `bltinsmodule.c` to Argument Clinic

# Feature or enhancement

There are several functions that are still not coverted to argument clinic (while they even say `AC: cannot convert yet, as needs PEP 457 group support in inspect`). I've found:
- `vars`
- `getattr`
- `dir`
- `next`
- `iter`

I will send a PR with these functions converted shortly.


### Linked PRs
* gh-100358