Heap types implemented in C must have Py_TPFLAGS_HAVE_GC and implement tp_traverse

If a heap type does not have Py_TPFLAGS_HAVE_GC or doesn't not implement tp_traverse, the garbage collector may fail to collect the type and most of its instances at Python exit. The problem is worse when sub-interpreters are involved: if a sub-interpreter exits and does not clear all of its memory, the other interpreters will continue running with these leaks.

See issue gh-87138 for the rationale.

I wrote an article on the lock type which had this bug and caused a memory leak: https://vstinner.github.io/subinterpreter-leaks.html

Linked PRs

• gh-116947
• gh-138290
• gh-138327
• gh-138328
• gh-138331
• gh-138333
• gh-138427
• gh-138428
• gh-138334
• gh-138340
• gh-138341
• gh-138582
• gh-138807
• gh-139073
• gh-139474
• gh-141268
• gh-141448
• gh-152310
• gh-156034

The following PRs were directly closed because the types are immutable and empty:

• gh-138329
• gh-138330
• gh-138332

The following PRs were wrongly merged so we reverted them:

• gh-138266
• gh-138320
• gh-138322
• gh-138288
• gh-138321
• gh-138323
• gh-138289
• gh-138324
• gh-138326

PRs for reverting the above commits:

• gh-138338
• gh-138337

Start with issue gh-87138 and the linked article about the lock type, then inventory the C heap types covered by the linked PRs. Done means applicable heap types have Py_TPFLAGS_HAVE_GC and tp_traverse, with the described interpreter-exit and sub-interpreter leaks addressed.