fix PIL/Pillow dep issue

[majidaldo]

from a clean slate, poetry installed mpl with an incompatible PIL. updating fixed it.

majidaldo@07014d0

maybe have mpl as an optional install if the user wants graphics. mpl brings the (complicated) scientific stack with it.

(you might even be able to knock out networkx but that's another discussion)

[gtfierro]

Python 3.7 is EOL in ~5 months but I'm not sure I want to remove support for it yet. I'd rather make mpl optional. Can you check that the linked PR #9 works for you?