Python 3 support

[MGatner]

I'm just getting into Sphinx so please forgive any ignorance... We're working on automating updates to a User Guide compiled with Sphinx (see e.g. codeigniter4/CodeIgniter4#2671 (comment)) and I was wondering if there's a hold up for switching the Action definition to use pip3 in support of Python 3 and the latest Sphinx versions?

[jreklund]

I took a closer look at the Dockerfile this project are using: python:3-slim.

And it's using Python 3 with pip. These Ubuntu builds are custom made, it's just the normal Ubuntu Server that have them separated into pip and pip3.

I think we can close this. @MGatner