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

[sobolevn]

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-100357: Convert several functions in bltinsmodule to AC #100358

[abdnafees]

What's an Argument Clinic?

[sobolevn]

@abdnafees you can read about it here: https://docs.python.org/3/howto/clinic.html :)

[abdnafees]

@sobolevn Thank you.