[docs] [issue33297] Mention Pillow package on tkinter.rst to work with more image formats

[Andrés Delfino]

Change by Andrés Delfino <adelfino at gmail.com>:


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keywords: +patch
pull_requests: +6201
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33297>
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