gh-130250: fix cleanup in test (impacts refleaks runs)

Author: iritkatriel

Lib/test/test_traceback.py

@@ -512,10 +512,13 @@ def test_print_exception_exc(self):
 
     def test_print_last(self):
         self.assertIsNone(getattr(sys, "last_exc", None))
-        sys.last_exc = ValueError(42)
-        output = StringIO()
-        traceback.print_last(file=output)
-        self.assertEqual(output.getvalue(), "ValueError: 42\n")
+        try:
+            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")