Update default Sphinx version

[limonkufu]

ReadtheDocs will work with a newer version of sphinx instead of 1.8.5 (over a year old)

Could we update the default sphinx version or give clear reasoning why we are using still the old versions?

I can work on this since we are using RTD a lot but I need a little bit of guidance

[stsewd]

You can use any version of sphinx you want https://docs.readthedocs.io/en/stable/guides/specifying-dependencies.html

The reason the default is 1.8.5 is because our theme doesn't support the latest version of sphinx yet (we are close to release a compatible version)

[limonkufu]

Thanks for the update. Is there anything I can do for the update process? Regardless, you can close this issue :)

[stsewd]

@limonkufu you can help us testing the pre-release version of the theme https://pypi.org/project/sphinx-rtd-theme/#history If you find a problem, please report that to https://github.com/readthedocs/sphinx_rtd_theme