Reference SPEC 0 in Python chapter

[sjvrijn]

SPEC 0 came up in the discussion around updating the supported versions for our Python template.
For consistency, I think it would make sense to refer to this in our guide too.

I think it could be mentioned under the "Dependencies and Package Management" header, since the SPEC literally covers "Minimally Supported Dependencies".

An alternative might be under "Use Python 3, avoid 2", since specifying support for main Python versions will be encountered by everyone making a package. Including it here would increase visibility since it'll be higher up on the page.

[bouweandela]

Would it make sense to replace "Use Python 3, avoid 2" with "Use a recent Python version" and then add a link to SPEC-0 under that? I think that Python 2 isn't used that much anymore.