Asciidoc transition to the python3 implementation or just EOL

[Joseph Herlant <aerostitch@debian.org>]

Hi guys,

As announces a while ago in #895462, the asciidoc package which only
support python2 is officially EOL.

Notes:
* The python 2 implementation of asciidoc which, for now, lives at
https://github.com/asciidoc/asciidoc won't receive any new updates
* The python 3 implementation that lives at
https://github.com/asciidoc/asciidoc-py3 started to have a working
version
* All the commits (and tags) that are in the python2 implementation
are currently in the python 3 implementation
* the binary names of the python3 implementation are exactly the same
as the python2 implementation (asciidoc and a2x)
* The upstream maintainers are currently working on clearly separating
the documentation for the asciidoc language from the python
implementation

There are currently 2 ways possible to handle the transition:
1. create a new package named asciidoc-py3 and have all the
dependencies moved to it
2. change the source of the asciidoc package to point to the
asciidoc-py3 repo (and create a new fake tag as upstream still didn't
cut a tag on it)
3. have people just move to asciidoctor (it's way more actively
maintained and tested, plus, most package support both nowadays) and
just let the python implementation die naturally

The 2nd implementation seems like an easier transition but might not
be the cleanest.

Any opinion?

Thanks,
Joseph