Fix docs environment to Python 3.12.

[freakboy3742]

The changes in #1940 didn't have the desired effect on dependabot (see #1937).

This takes a different approach - since RTD always uses Python3.12 as the build environment, we can force tox to also use Python3.12 as a development environment. This radically simplifies dependency configuration (as we only
need to worry about a single Python version), and eliminates a potential source of bugs (as different Python versions can render details like type annotations differently).

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

FWIW, while I philosophically support this change, I expect some subset of contributors to not understand how to satisfy this requirement....and it's usually non-trivial to explain to someone how to set up side-by-side Python installs 🫤

[rmartin16]

For consistency :) or perhaps the coverage section could be updated to match this

Suggested change

	basepython = python3.12
	base_python = py312

[freakboy3742]

WHY ARE THERE TWO VALID SYNTAXES FOR THIS!!>!!KJHKJH!OI&!^&!

[freakboy3742]

I agree this is a concern. However, the Sphinx ecosystem is making supporting "all valid Python versions" exceedingly difficult (deprecating Python 3.9 a full year before it goes EOL), and there are some advantages (such as being consistent with RTD rendering) that I think it's worth the pain.

If it turns out that it's more trouble than wrangling version dependencies, we can easily revert this and find another way to keep dependabot happy.

[mhsmith]

We now have 4 places referencing the Python version number. There's a very good chance that we'll forget one of them next time we have to update this.

Three of them are unavoidable: contribute-docs.rst, tox.ini and readthedocs.yaml. But they should each have a comment referencing the other two.

In pyproject.toml we can simply remove the expicit Python version number and change it to something like "Docs are always built on a specific Python version; see ...".