Fix SyntaxWarnings when building the docs

[JasonGrace2282]

• Bump readthedocs to use python 3.12 (so it's easier to confirm there being no SyntaxWarning without having to build it)
• Use raw strings to avoid syntax warnings
• Don't access the deprecated .tags attribute so that we can upgrade to Sphinx 9.0 without problems in the future

[behackl]

Curiously, this change apparently bumps up our documentation build time by about 2 minutes -- I can't really tell why.

Should we try to bump the version of Python directly to 3.13 to see whether it makes a difference?

[JasonGrace2282]

We could try bumping to 3.13. I'm not sure if that would make it faster, though - there was an issue with 3.13 that made it 30% slower with Sphinx. Python reverted the change, but it's possible it could still be slower.

I'm all for trying, though!

For reference, the build times are:
Python 3.11 - 16min 23s
Python 3.12 - 17min 56s

[JasonGrace2282]

It did get slightly faster with python 3.13, with 15min 45 seconds!