setup: explain to distutils how we write rc versions
When we use a rc version number (e.g. 4.8rc0), bdist_msi is using
distutils.StrictVersion to parse it into a tuple of numbers.
By default, StrictVersion.version_re only recognizes [ab] for alpha/beta,
where mercurial may use '-rc' or 'rc'.
This change makes StrictVersion parse correctly our version numbers, so that
bdist_msi doesn't fail on rc versions.
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors