differences to xarray version

[mathause]

I looked at the diff of minimum_versions.py to its counterpart in xarray and there are some minor differences. @keewis it's probably much easier for you to know if these should be ported here (or the other way round? They seem newer.).

[mathause]

We should also consider using this action for xarray - so they don't dis-align again.

[keewis]

yeah, I did add some changes to make the script possible to use as an action, but it looks like they diverged entirely when we pinned pydap in xarray.

I believe the additions in xarray might fix #19, we just have to figure out a way to pass the ignores to the action, and once the pinned version is within policy we might want to warn about the ignore, such that it is removed again.