TRIO100 does not detect async context managers

[CoolCat467]

Noticed this in my work on python-trio/trio#2947, it appears that TRIO100 does not detect async context managers.

minimal.py:

import trio

async def test_reentry_doesnt_deadlock() -> None:
    # Regression test for issue noticed in GH-2827
    # The failure mode is to hang the whole test suite, unfortunately.
    # XXX consider running this in a subprocess with a timeout, if it comes up again!

    async def child() -> None:
        while True:
            await trio.to_thread.run_sync(trio.from_thread.run, trio.sleep, 0, abandon_on_cancel=False)

    with trio.move_on_after(2):
        async with trio.open_nursery() as nursery:
            for _ in range(4):
                nursery.start_soon(child)

> ruff check minimal.py --isolated --select TRIO
minimal.py:12:5: TRIO100 A `with trio.move_on_after(...):` context does not contain any `await` statements. This makes it pointless, as the timeout can only be triggered by a checkpoint.

> ruff --version
ruff 0.2.1

[charliermarsh]

Thanks!