`ASYNC910/911`: require only one of `__aenter__`/`__aexit__` to contain a checkpoint

[Zac-HD]

Per https://trio.readthedocs.io/en/stable/reference-core.html#checkpoints

• If you call an async function provided by Trio (await <something in trio>), and it doesn’t raise an exception, then it always acts as a checkpoint. (If it does raise an exception, it might act as a checkpoint or might not.)
  • ...
  • Partial exception for async context managers: Both the entry and exit of an async with block are defined as async functions; but for a particular type of async context manager, it’s often the case that only one of them is able to block, which means only that one will act as a checkpoint.

So, we should not complain about missing checkpoints when they are part of an async context manager which does checkpoint in the other part. For example:

import trio

class CtxWithSetup:
    async def __aenter__(self):
        await trio.lowlevel.checkpoint()  # whatever slow setup

    async def __aexit__(self, exc_type, exc, tb):
        print(f"logging {exc=}")

class CtxWithTeardown:
    async def __aenter__(self):
        print(f"setup is quick")

    async def __aexit__(self, exc_type, exc, tb):
        await trio.lowlevel.checkpoint()  # whatever slow teardown

I wouldn't worry about inherited methods for now; we can handle those if someone requests them.