New versioning scheme

We’ve decided to change the version numbering scheme from the “semver” approach to a “date-based” approach, since that is better aligned with the practical usage of astroquery and will help us avoid the --pre requirement. Versions will look like `.YYYYMMDD`. For example, if we released one today, it would be `4.20191213`, and tomorrow would be `4.20191214`. When we increment the tag number, there will be a first tag that is just a bare number. The first one will therefore be 4.

This change will happen once we switch to using https://pypi.org/project/setuptools-scm/ for setup, since that supports setting these date-based versions.

Review the packaging setup when adopting setuptools-scm, starting with how the project currently defines its version. Confirm that releases produce the proposed .YYYYMMDD format, including the bare tag-number behavior, and verify the resulting package version before considering the work done.

Written by the indexing model from the issue text.