Autofix for missing checkpoints often puts checkpoints inside `except` blocks (violating `ASYNC120`)

[Zac-HD]

It'd be nice if our autofix didn't create new violations!

[jakkdl]

please give an example of a case where this happens so it's easier to get a start on.
Not infeasible, but probably somewhat complicated to fix given the complexity of Visitor91x.

[gml16]

Thanks John for taking a look. Here's a repro:

async def test():
    try:
        await trio.sleep(1)
        return
    except Exception:
        return

flake8-async --autofix=ASYNC file.py
gives

async def test():
    try:
        await trio.sleep(1)
        return
    except Exception:
        await trio.lowlevel.checkpoint()
        return