Switch to CalVer from SemVer?

[nstarman]

I think we can get the best of both with YYYY.MM.MICRO, which allows for fixes, e.g. updates to the documentation.
Now we just need a good backporting scheme... https://meeseeksbox.github.io
And commitizen supports custom bump rules (https://commitizen-tools.github.io/commitizen/customization/#custom-bump-rules) so we can "map" CalVer to SemVer, where YYYY.MM => Major, and MICRO => MICRO. commitizen can manage bumps, releases, and changelogs.