Use tox for development and consider supporting it

[nathanjmcdougall]

Motivation
At the moment, there is a significant amount of complexity in the codebase just with managing the Bitbucket Pipelines CI. I wonder whether tox is usually overkill for most projects, but it might be the right approach for usethis, because it makes it much easier to switch between CI providers (and makes supporting those providers much simpler). Bitbucket pipelines also has no notion of test matrices so it makes the syntax quite verbose which isn't ideal.

I think it would be good to add tox for development of this project and then see how the setup goes; are there pain points with the existing approach?

Will definitely want to use tox-uv.

Summary of desired enhancement
Consider using tox. Start by adding it for development, and consider the pros and cons for inclusion in usethis itself.