Handle attempts to "overwrite" interned heap types by static ones

Objects/unicodeobject.c

@@ -15022,6 +15022,29 @@ _PyUnicode_InternStatic(PyInterpreterState *interp, PyObject **p)
         return;
     }
 
+    PyObject *interned = get_interned_dict(interp);
+    if (interned) {
+        int res = PyDict_GetItemRef(interned, s, &r);
+        if (res < 0) {
+            PyErr_WriteUnraisable(*p);
+        }
+        else if (res == 1) {
+            // Someone "beat us to it"; there's an interpreter-specific key
+            // we must use :(
+            if (_Py_IsImmortal(r)) {
+                *p = r;
+            } else {
+                // Immortalize it.
+                // For now this is a slow path that re-does all the checks
+                // and lookups.
+                _PyUnicode_InternImmortal(interp, p);
+            }
+            _PyUnicode_STATE(*p).interned = SSTATE_INTERNED_IMMORTAL;
+            assert(_Py_IsImmortal(*p));
+            return;
+        }
+    }
+
     if (_Py_hashtable_set(INTERNED_STRINGS, s, s) < -1) {
         Py_FatalError("failed to intern static string");
     }