[pkg] add python3 only pypi specifiers
This is only for pypi statistics purposes.
Also add 'Programming Language :: Python', looks obvious but seems to be sane to have this as well.
Document: cubicweb-doc
Title: CubicWeb Documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developers
Section: Programming
Format: HTML
Index: /usr/share/doc/python3-cubicweb-documentation/html/index.html
Files: /usr/share/doc/python3-cubicweb-documentation/html/*