Support for matplotlib 3.8+ (positional to keyword args)

[kafitzgerald]

As of matplotlib 3.8 some positional arguments are no longer accepted. We use some of these (e.g. in NCL_polyg_19.py where we use Polygon).

This showed up in the upstream dev CI I set up. I'm guessing it's not showing up in other CI runs, because geocat-viz has matplotlib pinned to <3.8 until the next release.

Another reminder that upstream dev CI is probably a good idea.