switch to CalVer?

[keewis]

Our release policy very much follows that of xarray: deprecations will stay in a few releases, after which they will be applied (for example, removing old features or changing defaults).

This makes it somewhat difficult to choose the new version (when do we bump the which part of the version), and in my experience is often done somewhat arbitrarily. In addition, the version scheme is often confused with SemVer, which is not what we're doing.

With CalVer this is very easy: the version is always YYYY.0M.X, and if there's multiple releases per month we just have to increase X (for example, if there were bugs in the release).

As such, I would like to propose following xarray and switching to CalVer. What do you think?