[issue27100] Attempting to use class with both __enter__ & __exit__ undefined yields __exit__ attribute error

[Serhiy Storchaka]

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


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versions: +Python 3.7 -Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27100>
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