[issue30599] os.register_at_fork(): allow to unregister callbacks -- test_threaded_import leaks references

[Stéphane Wirtel]

Changes by Stéphane Wirtel <stephane at wirtel.be>:


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nosy: +matrixise

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30599>
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