Revert "Split test doesn't need to be executed per Python version (#130147)

[cdce8p]

Proposed change

This reverts commit a8db25f.

As explained in #130147 (comment), I believe it's safer to run the test collection once for every supported Python version, so that the collection and the actual test run are done with the same Python version.

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[cdce8p]

Not needed anymore, see #130147 (comment).