[issue45229] Always use unittest for collecting tests in regrtests

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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pull_requests: +27033
pull_request: https://github.com/python/cpython/pull/28668

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45229>
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