[issue21375] Fix and test overflow behavior in the C version of heapq

[Tim Peters]

Tim Peters added the comment:

I don't see a way to test it either, but it's easy enough to prove it's correct ;-)  That is, looks good to me!

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nosy: +tim.peters

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21375>
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