Over at the Trio project, we have evidence that `AsyncExitStack.enter_async_context(foo())` is not actually equivalent to `async with foo()` regarding raised exception context.

The symptom is a very long, unhelpful tracebacks because the __context__ of raised exceptions is not set to the expected object.

https://github.com/python-trio/trio/issues/2001

I can't speak to this solution myself, but njsmith suggests this amendment to contextlib:

    saved_context = exc_details[1].__context__
        try:
            raise exc_details[1]
        finally:
            exc_details[1].__context__ = saved_context