packaging: derive package version from SCM tag

In #63, I note that it's still necessary when cutting a release to remember to bump the version in the repo before tagging that same version. There exist tools out there to automatically derive the version from SCM metadata, which not only saves a step and avoids possible mismatches, but also has the benefit of creating intermediate versions (e.g. 0.47.0+g123abc) for untagged commits.