Python Environment: Use latest theme on all sphinx versions

[Blendify]

The latest theme version is compatible with all sphinx versions.
This should fix #8070

In the future, we will want to modify the version number used for old sphinx versions once sphinx 1.x is dropped by the theme.
This likely won't happen until sphinx-rtd-theme 2.0 see readthedocs/sphinx_rtd_theme#1086

I haven't tested this yet, but in the future, this may not be needed at all as we can handle dependency versions within the theme itself.

[stsewd]

New projects (projects created after Oct 20, 2020 ) use the latest version of sphinx and the rtd theme by default. This change will update the theme from 0.4.x to 0.5.x in projects created before oct 20, which may break some things. In the future we may stop installing the theme by default so users woul need to provide their requirements explicitily (#7859). I think is better if users update the theme explicitly or pin docutils to 0.16 explicitly https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html#don-t-rely-on-implicit-dependencies to fix this problem, otherwise upgrading may break things for some users.

[Blendify]

This change will update the theme from 0.4.x to 0.5.x in projects created before oct 20, which may break some things.

There shouldn't be any changes in updating the theme for these old projects. I looked over the changelog to make sure https://sphinx-rtd-theme.readthedocs.io/en/latest/changelog.html

[stsewd]

There shouldn't be any changes in updating the theme for these old projects. I looked over the changelog to make sure

I wasn't close to the 0.4 to 0.5 release, but if there isn't anything breaking I'm fine updating.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.