Supporting upstream supported Python versions

[jgart]

Hi Guixers,

What is the opinion on supporting current upstream supported versions
of python?

The master branch only has 3.9 but I'd like to have substitutes available
for 3.7, 3.8 and 3.10:

https://endoflife.date/python

Today, I needed python 3.8 and I had to resort to using asdf
(https://asdf-vm.com/) to get a binary. I'd prefer to use Guix for
getting all my needed python versions as substitutes. I think that providing
this is important if we want Guix to be a competitive tool in the python
ecosystem.

Could we make all upstream supported versions of python available in master?

This is currently the experience for ruby having various supported
versions in Guix master. I'd like to have available all supported python
versions also.

I'm happy to work on this.

wdyt