gh-135552: Don't clear weakref if wr_object is a type.

[nascheme]

This is an attempt at making a fix that could be backported to old Python versions. In main, I think the best fix is to stop clearing all weakrefs in the GC. However, that seems disruptive enough that we should avoid doing so in a bugfix release. This version of the fix doesn't clear if wr_object is a type and wr_callback == NULL. There is still a small chance this would break some code that expects that weakrefs are cleared by the GC. In the unit test suite, we had test_finalization, test_io and test_gc that was broken by not clearing weakrefs.

• Issue: Segmentation fault, possibly due to a GC issue (tp_subclasses) #135552

[nascheme]

I don't think this is safe actually, could introduce bugs like bpo-38006.