asciidoc: use python3

[paper42]

No description provided.

[ericonr]

There's a definitive version now: https://asciidoc.org/CHANGELOG.html

You could check if the output of doc builds is reasonable, because we definitely want to move to python3.

The commit message should look something like asciidoc: update to 9.0.0, use python3. Try cross building it as well, it might require the python_version variable too.

[sgn]

9.0.2 is released.

[paper42]

@sgn thanks, updated