Use python 3.12 in building docs, since no CuPy support for python 3.13

Author: antonwolfy

.github/workflows/build-sphinx.yml

@@ -36,7 +36,7 @@ jobs:
       pull-requests: write
 
     env:
-      python-ver: '3.13'
+      python-ver: '3.12'
       CHANNELS: '-c dppy/label/dev -c intel -c conda-forge --override-channels'
       NO_INTEL_CHANNELS: '-c dppy/label/dev -c conda-forge --override-channels'
       # Install the latest oneAPI compiler to work around an issue