[issue44206] Add a version number to dict keys.

[Mark Shannon]

Mark Shannon <mark at hotpy.org> added the comment:

http://theses.gla.ac.uk/2975/1/2011shannonphd.pdf page 128.

It means we don't need to cache a pointer to the keys, just the version number.
The version number is half the size (for 64 bit machines) and using it means that we don't leak keys.

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<https://bugs.python.org/issue44206>
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