[3.13] gh-140530: fix a reference leak in an error path for raise exc from cause (GH-140908) (#141283)

gh-140530: fix a reference leak in an error path for `raise exc from cause` (#140908)

Fix a reference leak in `raise E from T` when `T` is an exception
subtype for which `T.__new__` does not return an exception instance.

(cherry picked from commit 0c77e7c23b)

Python/ceval.c

@@ -1962,6 +1962,7 @@ do_raise(PyThreadState *tstate, PyObject *exc, PyObject *cause)
                               "calling %R should have returned an instance of "
                               "BaseException, not %R",
                               cause, Py_TYPE(fixed_cause));
+                Py_DECREF(fixed_cause);
                 goto raise_error;
             }
             Py_DECREF(cause);