Marshmallow 4.0.0 was just released

[tisdall]

https://marshmallow.readthedocs.io/en/latest/upgrading.html#upgrading-to-4-0

I haven't tried it yet, but I suspect things break since there's some major changes. I'd suggest changing the setup.py to have "marshmallow >= 3.19.0, < 4.0.0" to stay on the 3.0 branch until you've evaluated what needs to be changed here.

[swember]

Hello @tisdall, thanks for noticing this. I'll prepare an update for it.

[tisdall]

release 4.6.0 fixes this issue with #108