[tox] Do not install package in "develop"
This might shadow so packaging issues.
And user can still pass --develop to tox.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*