bpo-46417: Finalize structseq types at exit (GH-30645)

Add _PyStructSequence_FiniType() and _PyStaticType_Dealloc()
functions to finalize a structseq static type in Py_Finalize().
Currrently, these functions do nothing if Python is built in release
mode.

Clear static types:

* AsyncGenHooksType: sys.set_asyncgen_hooks()
* FlagsType: sys.flags
* FloatInfoType: sys.float_info
* Hash_InfoType: sys.hash_info
* Int_InfoType: sys.int_info
* ThreadInfoType: sys.thread_info
* UnraisableHookArgsType: sys.unraisablehook
* VersionInfoType: sys.version
* WindowsVersionType: sys.getwindowsversion()

Objects/structseq.c

@@ -532,6 +532,36 @@ PyStructSequence_InitType(PyTypeObject *type, PyStructSequence_Desc *desc)
     (void)PyStructSequence_InitType2(type, desc);
 }
 
+
+void
+_PyStructSequence_FiniType(PyTypeObject *type)
+{
+    // Ensure that the type is initialized
+    assert(type->tp_name != NULL);
+    assert(type->tp_base == &PyTuple_Type);
+
+    // Cannot delete a type if it still has subclasses
+    if (type->tp_subclasses != NULL) {
+        return;
+    }
+
+    // Undo PyStructSequence_NewType()
+    type->tp_name = NULL;
+    PyMem_Free(type->tp_members);
+
+    _PyStaticType_Dealloc(type);
+    assert(Py_REFCNT(type) == 1);
+    // Undo Py_INCREF(type) of _PyStructSequence_InitType().
+    // Don't use Py_DECREF(): static type must not be deallocated
+    Py_SET_REFCNT(type, 0);
+
+    // Make sure that _PyStructSequence_InitType() will initialize
+    // the type again
+    assert(Py_REFCNT(type) == 0);
+    assert(type->tp_name == NULL);
+}
+
+
 PyTypeObject *
 PyStructSequence_NewType(PyStructSequence_Desc *desc)
 {