[test/tox] Enforce installation of modified yapps 580a6821713d is not enough for tox apparently... The thing is that it will first install dependencies (the `deps` option from tox.ini), actually pulling yapps from Logilab github but then, it will install the project (that is, CubicWeb) from the sdist *along with its dependencies*, which will pull yapps again and override the previously installed version for some mysterious reason. So add a `pip install` command in tox.ini, so that the modified yapps is the last one installed.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
http://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))