[issue31033] Improve traceback of cancelled tasks / add cancel() msg argument

[Chris Jerdonek]

Change by Chris Jerdonek <chris.jerdonek at gmail.com>:


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pull_requests: +19472
pull_request: https://github.com/python/cpython/pull/20173

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31033>
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