The Python release process runs tests sequentially. It catchs some issues which are ignored silently when running tests in parallel.

I propose adding `--sequentially` option to regrtest to always run tests sequentially. It can be used to ignore any `-jN` option and also to re-run failed tests sequentially.

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### Linked PRs
* gh-119728
* gh-120456
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Add `--single-process` option to regrtest to always run tests sequentially (ignore `-jN` option)
This is a GitHub Issue
repo:cpython
owner:python
Title : Add `--single-process` option to regrtest to always run tests sequentially (ignore `-jN` option)
Issue date:

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The Python release process runs tests sequentially. It catchs some issues which are ignored silently when running tests in parallel.

I propose adding `--sequentially` option to regrtest to always run tests sequentially. It can be used to ignore any `-jN` option and also to re-run failed tests sequentially.

<!-- gh-linked-prs -->
### Linked PRs
* gh-119728
* gh-120456
<!-- /gh-linked-prs -->

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