Issue #27123: When an exception is raised within the context being

managed by a contextlib.ExitStack() and one of the exit stack
generators catches and raises it in a chain, do not re-raise the
original exception when exiting, let the new chained one through.
This avoids the PEP 479 bug described in issue25782.

diff --cc Lib/contextlib.py
Simple merge

diff --cc Lib/test/test_contextlib.py
Simple merge

diff --cc Misc/NEWS
Simple merge