ASYNC103 for sync functions

[A5rocks]

I don't think sync functions can raise trio.Cancelled, but this is causing ASYNC103:

    def call(
        self,
        hookname: str,
        *args: object,
    ) -> None:
        # snip
        for instrument in list(self.data[hookname]):
            try:
                getattr(instrument, hookname)(*args)
            except BaseException:  # E
                self.remove_instrument(instrument)
                INSTRUMENT_LOGGER.exception(
                    "Exception raised when calling %r on instrument %r. "
                    "Instrument has been disabled.",
                    hookname,
                    instrument,
                )

Error message:

src/trio/_core/_instrumentation.py:110:20: ASYNC103 BaseException block with a code path that doesn't re-raise the error. Consider adding an `except trio.Cancelled: raise` before this exception handler.

[Zac-HD]

They can't originate a Cancelled, but can definitely propagate it; and iirc the instruments interface can sometimes hit this even with idiomatic use. (mostly it's a protection for weird code that I keep seeing in practice though)

[A5rocks]

Hm, propagate from what? I think trio.run shouldn't ever raise Cancelled...

(I'm probably just forgetting that case!)

[Zac-HD]

pretty sure you can get Cancelled coming out of threads? I definitely agree that this "mostly shouldn't happen", but the simplicity/consistency argument plus weird edge cases has me inclined to keep it anyway.

[jakkdl]

I can see a case for splitting the error for sync functions into a different error code. I don't remember if async103 is gated to only trigger in files importing trio, but for a big code base that only uses trio for part of it it could be very noisy.
(we also don't have native --per-file-ignore settings, and using flake8's might be flaky #310)

[A5rocks]

Oh on further thought, the only situation I can think of this happening is something like @contextmanager. So it can happen, but presumably only in functions with a yield?

(FWIW to provide some color, this error happens ~5 times in Trio, but seems useful so I'm not sure I should disable it...)