[3.9] bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089) (GH-28731)

[3.9] bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089) (GH-28731)

Make enter_context(foo()) / enter_async_context(foo()) equivalent to
`[async] with foo()` regarding __context__ when an exception is raised.

Previously exceptions would be caught and re-raised with the wrong
context when explicitly overriding __context__ with None..
(cherry picked from commit e6d1aa1ac65b6908fdea2c70ec3aa8c4f1dffcb5)

Co-authored-by: John Belmonte <john@neggie.net>
Automerge-Triggered-By: GH:njsmith