[issue44657] instancemethod_call should use PyInstanceMethod_GET_FUNCTION macro

[Dong-hee Na]

Change by Dong-hee Na <donghee.na at python.org>:


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versions:  -Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44657>
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