Add another way for extensions to register static constants

well, ones that aren't quite constant.  They can just register
their C static variables and then change them at will.

The _warnings module does this

from_cpython/Include/Python.h

@@ -135,6 +135,7 @@ PyAPI_FUNC(void) PyType_SetDict(PyTypeObject*, PyObject*) PYSTON_NOEXCEPT;
 // get decref'd when the interpreter shuts down.  This functions returns its argument.
 // PyType_Ready calls this automatically.
 PyAPI_FUNC(PyObject*) PyGC_RegisterStaticConstant(PyObject*) PYSTON_NOEXCEPT;
+PyAPI_FUNC(void) PyGC_RegisterStaticConstantLocation(PyObject**) PYSTON_NOEXCEPT;
 
 // Gets gc.garbage
 PyAPI_FUNC(PyObject*) _PyGC_GetGarbage(void) PYSTON_NOEXCEPT;

from_cpython/Python/_warnings.c

@@ -538,8 +538,8 @@ setup_context(Py_ssize_t stack_level, PyObject **filename, int *lineno,
                     goto handle_error;
                 }
                 else if (!is_true) {
-                     Py_DECREF(*filename);
-                     *filename = PyString_FromString("__main__");
+                    Py_DECREF(*filename);
+                    *filename = PyString_FromString("__main__");
                     if (*filename == NULL)
                         goto handle_error;
                 }
@@ -901,7 +901,7 @@ _PyWarnings_Init(void)
     _filters = init_filters();
     if (_filters == NULL)
         return;
-    PyGC_RegisterStaticConstant(_filters);
+    PyGC_RegisterStaticConstantLocation(&_filters);
     Py_INCREF(_filters);
     if (PyModule_AddObject(m, "filters", _filters) < 0)
         return;

src/runtime/types.cpp

@@ -4091,7 +4091,12 @@ void BoxedGetsetDescriptor::dealloc(Box* _o) noexcept {
 #endif
 
 std::vector<Box*> constants;
+std::vector<Box**> constant_locations;
 std::vector<Box*> late_constants;
+extern "C" void PyGC_RegisterStaticConstantLocation(Box** ptr) noexcept {
+    constant_locations.push_back(ptr);
+}
+
 extern "C" PyObject* PyGC_RegisterStaticConstant(Box* b) noexcept {
     constants.push_back(b);
     return b;
@@ -4827,6 +4832,11 @@ extern "C" void Py_Finalize() noexcept {
             break;
     }
 
+    for (auto p : constant_locations) {
+        Py_DECREF(*p);
+    }
+    constant_locations.clear();
+
     clearAllICs();
     PyType_ClearCache();
     PyOS_FiniInterrupts();
@@ -4851,6 +4861,7 @@ extern "C" void Py_Finalize() noexcept {
     while (PyGC_Collect())
         ;
     assert(!constants.size());
+    assert(!constant_locations.size());
 
     _Py_ReleaseInternedStrings();