tests/cpydiff: Fix test case for modules_json_nonserializable.

The test case was producing the following error:

    Traceback (most recent call last):
      File "<stdin>", line 12, in <module>
    UnicodeError:

which did not demonstrate the intended difference (this particular
non-json-serializable object DID throw an exception! just not TypeError).

The updated test uses a byte string with all ASCII bytes inside, which
better illustrates the diference.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/cpydiff/modules_json_nonserializable.py

@@ -6,10 +6,7 @@ workaround: Unknown
 """
 import json
 
-a = bytes(x for x in range(256))
 try:
-    z = json.dumps(a)
+    print(json.dumps(b"shouldn't be able to serialise bytes"))
-    x = json.loads(z)
-    print("Should not get here")
 except TypeError:
     print("TypeError")