test: xfail a test because of CPython issue 104686

Author: nedbat

tests/test_arcs.py

@@ -1700,6 +1700,10 @@ def my_function(
         )
 
     @xfail_pypy38
+    @pytest.mark.xfail(
+        env.PYVERSION > (3, 12, 0, "alpha", 7, 0),
+        reason="Pending bug https://github.com/python/cpython/issues/104686"
+    )
     def test_class_decorator(self) -> None:
         arcz = (
             ".1 16 67 6D 7A AE E. "     # main line