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

[Tim Peters]

Tim Peters <tim at python.org> added the comment:

Stephane, it's not deep.  People who need to write doctests that work across N versions of Python shouldn't need to read N versions of the documentation.  This is hardly unique to doctest.  We routinely add "Changed in version m.n" blurbs all over the place.

Ways that were _necessary_ for robust dict testing continue to work fine in 3.6 and 3.7, so it doesn't harm anything if people mindlessly copy an example that _could_ be spelled some other way under 3.6+.  It's not like there's even anything slightly obscure about, e.g.,

>>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
True

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