[issue29822] inspect.isabstract does not work on abstract base classes during __init_subclass__

[Serhiy Storchaka]

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


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stage: patch review -> backport needed
versions:  -Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29822>
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