python 3.8 compat

[mircealungu]

Would it be too much to ask also for Python 3.8 compatibility?

On one of my home machines it seems I still have that one installed.

Obviously it's not about me, but I'm thinking about possible users, 3.8 is still supported till the end of the year officially (https://devguide.python.org/versions/)

Obviously, if this is not a big hassle. If it is, then we'll argue that one of the beneficial side-effects of mt-diagrams is that it forces people to upgrade their python :)