Update docs building dependencies and configs

[dsagal]

• Use pinned and up-to-date dependencies for generating documentation
• Regenerate sphinx config file, and documentation on readthedocs
• Preview available at https://asttokens.readthedocs.io/en/sphinx-conf/

[alexmojaki]

The docs preview is missing ASTText. Did you build it locally without updating master?

[dsagal]

The issue is that auto-documentation isn't fully automatic, but only selected classes and members are included. So I have to list ASTText explicitly. Including :members: at top level would show that automatically, but in a differently organized layout, so I added ASTText explicitly to keep layout as before. One top-level member not documented is supports_tokenless. Not sure it's worth documenting as a top-level function. Maybe could be moved to util? Anyway, that seems separate from this task.

[alexmojaki]

I don't know what most of this stuff means 🤷

Also I still don't see ASTText in the preview, did you rebuild?

[dsagal]

Sorry, I hadn't realize that rebuilding from branches wasn't automatic.

[alexmojaki]

https://asttokens.readthedocs.io/en/stable/api-index.html still needs updating

[dsagal]

I updated a setting, it rebuilt, and looks up-to-date now. It should be automatic, from skimming https://docs.readthedocs.io/en/stable/versions.html. Maybe it was the missing setting. If it doesn't auto-update after next release, I'd have to debug further.