Alternative version release management

trying out an alternative. we're using a ``__VERSION__`` variable in the setup.py, and relying on bumpversion (or manual patch) to bump the given version up-to-date. glitches I see:

- we can't always be sure the CHANGELOG file is up to date,
- external modules (sphinx doc, etc) can't directly access the version variable. is it really an issue? as far as I can tell, RTFD doesn't really use the sphinx configuration, but relies on the git tags,

would it be possible to get this information somewhere in the source code of workalendar? (e.g. in the workalendar/__init__.py file?) or is it a bad idea?