Replace versioning with bump2version

[agjohnson]

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A neat idea was brought up in a theme PR: https://github.com/readthedocs/sphinx_rtd_theme/pull/822

It would replace code that we have for loading the version from setup.cfg perhaps, but would also replace the invoke code around cutting a release.

[agjohnson]

I've recently come across bumpver, which solves many issues I've had with bump2version. I have changes in to swap over our deploy scripts and repos, it's mostly a drop in replacement, and we aren't required to manually increment version parts anymore. It just manages all of the various version strings we have littered around each repo.

• https://github.com/readthedocs/readthedocs.org/pull/9029
• https://github.com/readthedocs/readthedocs-corporate/pull/1392