Since it turns out there's even case(s) in `trio` core that are guilty
(of implementing things like checkpoints in exc handlers), this adds
facility for ignoring explicit cases via `inspect.FrameInfo` field
matching from the unmasked `exc_ctx` within
`maybe_raise_from_masking_exc()`.
Impl deats,
- use `inspect.getinnerframes()/getmodule()` to extract the equivalent
"guilty place in code" which raised the masked error which we'd like
to ignore and **not unmask**.
- start a `_mask_cases: dict` which describes the entries to ignore
by matching against a specific `FrameInfo`'s fields from indexed
from `getinnerframes()`.
- describe in that table the case i hit with `trio.WouldBlock` being
always masked by a `Cancelled` due to way `trio.Lock.acquire()`
implements the blocking case in the would-block handler..
- always call into a new `is_expected_masking_case()` predicate (from
`maybe_raise_from_masking_exc()`) on matching `exc_ctx` types.
79eb9be69f