[issue41431] Optimize dict_merge for copy

[Inada Naoki]

Change by Inada Naoki <songofacandy at gmail.com>:


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pull_requests: +20819
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/21674

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41431>
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