bpo-44594: fix (Async)ExitStack handling of __context__ (gh-27089)

* bpo-44594: fix (Async)ExitStack handling of __context__

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.