change python-docutils build dependency to python3

What does this MR do and why?

Just noticed, that the python-docutils build dependency in the source install docs no longer exists on the current Debian stable (bullseye), only the python 3 version does.

Since I've been running the source installation on bullseye successfully for over a year now, one might wonder, though, whether this dependency is actually needed. :)

• https://packages.debian.org/search?keywords=python%2Ddocutils (you see that packaging ends with buster)
• https://packages.debian.org/search?keywords=python3%2Ddocutils (still packaged)

How to set up and validate locally

1. Try to run apt install python-docutils on Debian bullseye
2. Get a Package 'python-docutils' has no installation candidate error.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.