Make strip_extra a direct class property in the test

Author: danny0838

Lib/test/test_zipfile/test_core.py

@@ -5840,9 +5840,7 @@ class StripExtraTests(unittest.TestCase):
 
     ZIP64_EXTRA = 1
 
-    @classmethod
-    def setUpClass(cls):
-        cls.strip_extra = staticmethod(zipfile.ZipFile._strip_extra_fields)
+    strip_extra = staticmethod(zipfile.ZipFile._strip_extra_fields)
 
     def test_no_data(self):
         s = struct.Struct("<HH")