Allow ranges in generative environment list

[mimre25]

What's the problem this feature will solve?

When supporting multiple versions, eg python 3.8 to python 3.13, I noticed myself always going with an environment definition a la:

[tox]
env_list = py3{8,9,10,11,12,13}

Which is a bit tedious and wordy.

Describe the solution you'd like

Before typing the above, I usually go with

[tox]
env_list = py3{8-13}

just to find out that this doesn't work.

I'd really like to have these ranges in there if possible.
It is terser and follows the natural reading flow of "from python 3.8 to python 3.13" instead of "python 3.8, python 3.9, python 3.10 ...".

Additional context

Happy to contribute a PR if I get a go-ahead. 🙂

[gaborbernat]

Go ahead.