versioning packages should render version

[edublancas]

We want to include the next release version in the source code in a few cases. For example, when using .. versionadded::, .. versionchanged::, or when deprecating features. However, the code author might not know which the next release.

The idea is to have placeholders that render when creating a new version. e.g.,

warn("X was renamed to Y in version {VERSION_NEXT} and X will be removed in {VERSION_DEPRECATE}", FutureWarning)

We should also keep track of {VERSION_DEPRECATE} so when we reach such version, we do not forget to deprecate them. Maybe we can store a .deprecations JSON file that keeps track of these things.