[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.

[MESSAGES CONTROL]
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[VARIABLES]
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[BASIC]
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[TYPECHECK]
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