Pin Python version (or update Sphinx)

[mrienstra]

Python 3.13 breaks sphinx.builders.epub3, due to the removal of imghdr, see sphinx-doc/sphinx#10440

This can be resolved by upgrading to Sphinx v6.2.0 or newer (latest version is 8.2.3). This project currently uses sphinx 4.5.0.

Aside: myst-parser 0.18.1 won't work with sphinx 6+, latest version is 4.0.1. Probably many other dependencies would also need to be bumped. The current default theme (sphinx_rtd_theme) would probably need to jump from 1.3.0 to a newer release, latest is 3.0.2.

---

pyproject.toml:

[tool.poetry.dependencies]
python = "^3.8"

.readthedocs.yml:

  tools:
    python: "3.11"

... so obvious simple fix would be to set both of these to the same specific version, 3.11 or 3.12.