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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).