Checker `async.py` couldn't be imported anywhere because it uses reserved word.

[bgb10]

Current problem

Wanted to make test code for async checkers, but since name of async checker is async.py, AsyncChecker class in this file cannot be imported anywhere. ('async' is keyword!) Indeed, we imported this nowhere before (just registered checker to linter as below), but to write test code for AsyncChecker, file rename is must.

Desired solution

So renamed async.py to async_checker.py.

Additional context

PR: #10060

[bgb10]

I created PR at first, and made this to write changelog 😄

[Pierre-Sassoulas]

Thank you. You can refs the pull request directly next time :)