False alarm for `TRIO103`, on `except BaseException` after an `except Cancelled` clause

[Zac-HD]

try:
    ...
except trio.Cancelled:
    raise
except BaseException:
    pass  # this should be OK, because the error _can't_ be a Cancelled.

We should also make the README and the message for BaseException suggest this pattern.

[jakkdl]

Oh goodness, so spammy if you're amending a commit that mentions the issue