test: 3.10.0b4 traces match/case incorrectly

See: https://bugs.python.org/issue44600

Author: nedbat

tests/test_arcs.py

@@ -1215,7 +1215,11 @@ def gen():
         )
 
 
-@pytest.mark.skipif(not env.PYBEHAVIOR.match_case, reason="Match-case is new in 3.10")
+three_ten_not_ready = (env.PYVERSION <= (3, 10, 0, 'beta', 4, 0))
+@pytest.mark.skipif(
+    three_ten_not_ready or not env.PYBEHAVIOR.match_case,
+    reason="Match-case is new in 3.10",
+)
 class MatchCaseTest(CoverageTest):
     """Tests of match-case."""
     def test_match_case_with_default(self):