replace async-timeout with asyncio.timeout

[dotlambda]

[mosquito]

Hi @dotlambda, could you please explain what exactly problem you're solving with this PR? I don't understand why we should drop support for Python 3.10, whose EoL is in a year. If you don't mind, perhaps we could wait with merging until then.

[dotlambda]

I'm just trying to follow upstream's recommendation.

We could also use

if sys.version_info >= (3, 11):
    from asyncio import timeout
else:
    from async_timeout import timeout

[mosquito]

It was a dev dependency, needed only for tests.