gh-124771: Add the PYTHON_GC_PRESET env var.

[nascheme]

This is still a WIP. We need to discuss if this is actually a good idea and what the options should be if it is.

There should be a gc.set_preset() function, which is not yet implemented. Also need unit tests for this functionality. The initconfig logic is fairly complex and I'm not sure I did everything correct.

• Issue: Add a high-level tuning option for the cyclic GC #124771

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📚 Documentation preview 📚: https://cpython-previews--124772.org.readthedocs.build/

[vstinner]

The name PYTHON_GC_PRESET was suggested.

[nascheme]

The name PYTHON_GC_PRESET was suggested.

Yeah, that's a better name. I've updated the PR. Still need the gc.set_preset() function and some unit tests before this would be ready to merge.

[vstinner]

The PR is a draft. Does it mean that you're not sure if it's a good idea or not? Should it be reviewed or not yet?

[nascheme]

The code is not yet complete (need gc.set_preset() and gc.get_preset()). Also, I'm hoping for some more discussion on the approach, both from other Python implementations like pypy and from GC experts like Joannah.