Update to sphinx 8 on October 7th

[cthoyt]

Update to sphinx 8 when RTD 3.0 theme that supports sphinx 8 comes out

readthedocs/sphinx_rtd_theme#1598 (comment)

[cthoyt]

See https://about.readthedocs.com/blog/2024/07/addons-by-default/, probably need to add the following to conf.py:

import os

# Define the canonical URL if you are using a custom domain on Read the Docs
html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")

# Tell Jinja2 templates the build is running on Read the Docs
if os.environ.get("READTHEDOCS", "") == "True":
    if "html_context" not in globals():
        html_context = {}
    html_context["READTHEDOCS"] = True