[MRG] Update to py 3.13 for doc build

[DoomedJupiter]

Update the python version used to build the docs. Change made in accordance with rtd config.

Supports PR #551

[bosd]

Oops, Maybe I approved too quickly.
Docs-build and xdoctest still mentions python 3.10
Will look later into this, when more time.

[DoomedJupiter]

Updated the py version used to run the docs-build test in tests.yml. Also updated noxfile.py to use py v3.13. I don't quite understand the syntax here as it looks like we are forcing the py version with python_verion[5] even though tests.yml appears to be pushing py v3.13.

The test xdoctest looks to be some sort of test on docstrings using the xdoctest package. It seems like this is not related to the sphinx bump so I left it as is, using py v3.10