Skip test_assertion_walrus_different_test_cases on Python 3.7

Author: nicoddemus

testing/test_assertrewrite.py

@@ -1532,6 +1532,7 @@ def test_gt():
 
 
 class TestIssue11239:
+    @pytest.mark.skipif(sys.version_info[:2] <= (3, 7), reason="Only Python 3.8+")
     def test_assertion_walrus_different_test_cases(self, pytester: Pytester) -> None:
         """Regression for (#11239)