[issue28183] Clean up and speed up dict iteration

[INADA Naoki]

INADA Naoki added the comment:

Draft Misc/NEWS entry (and commit message) is:

 Core and Builtins
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+- Issue #28183: Optimize dict iteration.
+

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28183>
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