Add PYTHON_GC_THRESHOLD env var

[nascheme]

Feature or enhancement

Proposal:

It would be convenient sometimes to set the GC threshold using an env variable rather than calling gc.set_threshold().

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• GH-124742: Add the PYTHON_GC_THRESHOLD environment variable. #124743