bpo-29854: Skip history-size test on older readline (GH-2621)

Turns out that history-size was added in readline 6.0. This explain why
this tests fail on FreeBSD when using readline 5.2. We skip now the
history size if readline does not support it.

See https://cnswww.cns.cwru.edu/php/chet/readline/CHANGES for
details.