bpo-30621: Update Input Output Tutorial Example(GH-2074)

import json before using json module

Doc/tutorial/inputoutput.rst

@@ -412,6 +412,7 @@ sent over a network connection to some distant machine.
 If you have an object ``x``, you can view its JSON string representation with a
 simple line of code::
 
+   >>> import json
    >>> json.dumps([1, 'simple', 'list'])
    '[1, "simple", "list"]'