[docs] [issue34839] doctest: Change example under warnings section

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

To me, the question is whether to delete the dict example, or qualify it with "Before 3.7 (or 3.6 for CPython)" but leave the illustration of workarounds. Raymond, what do you thing?

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nosy: +rhettinger, terry.reedy

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