c.f. python-trio/trio#455 (comment) - this would be similar to ASYNC102
but I think ideally a separate rule so that people who wish to disable this (if lost exceptions aren't a big deal for them) don't reintroduce lost cancellations.
We should probably also do something similar for __aexit__
methods, although this gets increasingly awkward for @asynccontextmanager
functions and so that should probably be a further separate rule.