"characters()" now issues the new "NonRoundTrippableCharactersWarning"
if the "codec" argument allows generating characters which encode
successfully but do not decode back to the same character - for
example the yen sign becomes a backslash under "shift_jis" - since
strings containing them do not round-trip (issue #4813). Pass each
such character in "include_characters" to generate it without the
warning, or in "exclude_characters" to generate only characters which
round-trip.
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