[BUG] Heap types created by type specs don't handle GC

[scoder]

As noted in #3611 (comment), heap types need to implement the GC protocol, tp_traverse and tp_clear. When the CYTHON_USE_TYPE_SPECS option is set, this applies to all extension types.

See https://bugs.python.org/issue42972

[da-woods]

I'm going to do this once #5846 is merged. It's slightly simpler than I was giving it credit for - only tp_traverse needs to handle type(self) (not tp_clean) and I think Python does the right thing for classes that only need to handle type(self). The only complication is that it's version dependent.