Consider dropping marshmallow_enum from dependencies

The project seems to be abandoned (no change since 2019), and has potential to break things in the future, with PRs like this https://github.com/justanr/marshmallow_enum/pull/49 pending.

Something along the lines of https://github.com/marshmallow-code/marshmallow/issues/1470#issuecomment-563114297 might be applicable, though haven't tried to make it work on this codebase much yet.