Do not test Attr.specified, which is only set in 3.16

Author: serhiy-storchaka

Lib/test/test_minidom.py

@@ -956,7 +956,6 @@ def testParseAttributes(self):
         self.assertTrue(elem.hasAttribute("a"))
         self.assertFalse(elem.hasAttribute("missing"))
         attr = elem.getAttributeNode("a")
-        self.assertTrue(attr.specified)
         self.assertIsNone(attr.namespaceURI)
         dom.unlink()