Fix code-object reference cycles in GIL-enabled builds by making code objects participate in cyclic garbage collection.

The change uses GC allocation and deallocation for code objects in all builds, enables tp_traverse, and visits co_consts. It also adds a regression test that verifies a cycle from co_consts back to the code object is collected.

Closes #148722.

Tests:

• make -j8
• ./python.exe -m test test_code test_marshal test_gc
• Original issue reproducer

• Issue: Reference leak in code object with reference loop #148722