[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.