Build docs on 3.12

The Sphinx error is the same as the flake8 error on Python 3.11. Changing the build version on readthedocs should fix the problem.

I also now need to install the package for readthedocs (as I import it in `conf.py` to exclude some symbols). I've added
`python.install.path: .` which I hope will do that.

Author: rwb27

.readthedocs.yaml

@@ -8,7 +8,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "3.12"
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx:
@@ -25,3 +25,4 @@ sphinx:
 python:
   install:
     - requirements: dev-requirements.txt
+    - path: .