Add an "assert mode" to `-X showrefcount` that fails when references leak

[encukou]

The -X showrefcount option currently prints the number of references at exit.
Once we get the number to 0, we should test that it stays that way. It would be nice to add a mode, say -X showrefcount=assert0 or -X checkrefcount, that exits with nonzero if refs/blocks remain. Then it can be used in subprocess tests.

cc @vstinner

[jkloth]

There is an existing test for 0 leaked refcounts: test_embed.test_no_memleak.

It currently only ensures that leaks do not occur within just an init/fini cycle.

[vstinner]

I'm not sure if such feature would be useful today, since there are still many Python stdlib extensions which leak memory at exit :-(

• Convert static types to heap types: use PyType_FromSpec() #84258
• Clear static types in Py_Finalize() for embedded Python #90575 (comment)

Well, at least last time I checked. Maybe things went better since I checked.

[encukou]

Well, there are some extensions/tests that don't leak memory, and the option could be used to ensure those don't start leaking.