More robust version handling: Take 2: Taking max version as "latest"

[thorwhalen]

First read More robust version handling. The current issue is a continuation of this.

The solution implemented to close #11 was to separate some concerns, cleaning things up a bit, and introducing a versions_from_different_sources which would then be validated via the validate_versions.

The idea was to get earlier failures when the different versions weren't aligned (which would cause later on harder failures to debug). And from there, the idea was to change the mechanics of how versions are created to avoid divergence in the first place. This would be the cleaner solution. We'll do that maybe, one day. See #30 for this.

But we don't have time to look into a cleaner solution, so instead we'll try to implement a solution focusing on reducing current and future failures.

The proposed solution is:

Demote validation to something softer:
Change validate_versions so we can control the action when invalid (to be able to raise, ignore, warn etc.), and then, in get_new_version, use it with a action_when_versions_not_valid=warn default:

    validate_versions(versions, action_when_not_valid=action_when_versions_not_valid)

Use the max of all the version "opinions" as the "current" (or rather "latest") version that will be bumped.

    # Take the highest version from the different sources to be the latest version
    latest_version = max(filter(None, versions.values()), key=semver.VersionInfo.parse)