gh-117142: ctypes: Fix memory leak of StgInfo

[neonene]

Fix intentional memory leak during a finish by introducing _PyType_GetBaseBySpec() with a type slot ID.

Discussion: encukou/abi3#19

>python_d -X showrefcount -c "import ctypes"
[188 refs, 132 blocks]  # before
[0 refs, 0 blocks]      # after

cc @encukou

• Issue: Convert _ctypes extension module to multi-phase init #117142

[encukou]

All metaclasses defined by _ctypes derive from PyCType_Type. So, if there are mro entries with the right ModuleDef, the last one is PyCType_Type.

[encukou]

Thank you for working on this. It clearly shows we need better APIs to handle type/module teardown correctly in general.
For a stdlib module, in 3.13, we can use implementation details for a less invasive fix: #119424.