[issue40221] Use new _at_fork_reinit() lock method in multiprocessing

[Dong-hee Na]

Dong-hee Na <donghee.na92 at gmail.com> added the comment:

we should find all usages from Lib/multiprocessing/*.py

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40221>
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