Issue19619: skip zlib error test when zlib not available

Lib/test/test_codecs.py

@@ -2335,7 +2335,7 @@ transform_aliases = {
 try:
     import zlib
 except ImportError:
-    pass
+    zlib = None
 else:
     bytes_transform_encodings.append("zlib_codec")
     transform_aliases["zlib_codec"] = ["zip", "zlib"]
@@ -2440,6 +2440,7 @@ class TransformCodecTest(unittest.TestCase):
                     bad_input.decode("rot_13")
                 self.assertIsNone(failure.exception.__cause__)
 
+    @unittest.skipUnless(zlib, "Requires zlib support")
     def test_custom_zlib_error_is_wrapped(self):
         # Check zlib codec gives a good error for malformed input
         msg = "^decoding with 'zlib_codec' codec failed"