[issue29922] error message when __aexit__ is not async

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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keywords: +patch
pull_requests: +6061
stage: needs patch -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue29922>
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