Auto update of version pins based on upstream requirements #35

As asked by @weaverba137 over at astropy/astropy-project#255 (comment)

Is there a way to know what upstream dependencies Astropy currently supports (equivalently: is being tested against), independently of tracking down the testing configuration files and reading those? If that did exist, one could conceive of packaging it in e.g. JSON or YAML format, then dropping those versions into templates of e.g. setup.cfg, tox.ini, .readthedocs.yml, ci_workflows.yml, etc.

Note: "Astropy" could be generalized to any upstream Python dependency that has similar layout as the template, perhaps.