Change supported python versions to 3.8-3.12

[jasle]

Update supported python version to 3.8-3.12 and adjust checks accordingly.

[lovasoa]

I'm ok with dropping support for python 3.6 and 3.7

Together they represent <1.5% of downloads

https://pypistats.org/packages/marshmallow-dataclass

But can you please bump the major version, since this is a breaking change ?

[jasle]

But can you please bump the major version, since this is a breaking change ?

Raise it to 9.0.0 right?

[lovasoa]

Yes, this should be 9.0.0