[docs] [issue33283] Mention PNG as a supported image format by Tcl/Tk

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

The minimal supported version is 8.4 in 3.x and 8.3 in 2.7, which don't support PNG.

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components: +Tkinter
nosy: +serhiy.storchaka, terry.reedy
versions: +Python 2.7, Python 3.6, Python 3.7

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