[docs] [issue30621] import json before using json module in tutorial Input and Output

[Mariatta Wijaya]

Changes by Mariatta Wijaya <mariatta.wijaya at gmail.com>:


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stage:  -> patch review
type: resource usage -> 
versions: +Python 2.7, Python 3.5, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30621>
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