gh-130250: use support.swap_attr() in test and don't assume we're beginning with a clean system state (#130342)

Author: iritkatriel

Lib/test/test_traceback.py

@@ -511,14 +511,10 @@ def test_print_exception_exc(self):
         self.assertEqual(output.getvalue(), "Exception: projector\n")
 
     def test_print_last(self):
-        self.assertIsNone(getattr(sys, "last_exc", None))
-        try:
-            sys.last_exc = ValueError(42)
+        with support.swap_attr(sys, 'last_exc', ValueError(42)):
             output = StringIO()
             traceback.print_last(file=output)
             self.assertEqual(output.getvalue(), "ValueError: 42\n")
-        finally:
-            sys.last_exc = None
 
     def test_format_exception_exc(self):
         e = Exception("projector")