Remove binding of captured exceptions when not used to reduce the chances of creating cycles (GH-17246)

Capturing exceptions into names can lead to reference cycles though the __traceback__ attribute of the exceptions in some obscure cases that have been reported previously and fixed individually. As these variables are not used anyway, we can remove the binding to reduce the chances of creating reference cycles.

See for example GH-13135

Lib/test/test_cgitb.py

@@ -31,7 +31,7 @@ class TestCgitb(unittest.TestCase):
     def test_text(self):
         try:
             raise ValueError("Hello World")
-        except ValueError as err:
+        except ValueError:
             text = cgitb.text(sys.exc_info())
             self.assertIn("ValueError", text)
             self.assertIn("Hello World", text)