[pyramid] Rename paste app factory to pyramid_main Making it clearer that this would build a Pyramid application, just in case we eventually provide other means to create a WSGI application than Pyramid.

[tox] Do not use site packages in testenv This is the normal way of running tox. We used to set this because some people liked to run tests using packages installed by other means (i.e. system-wide). Now, there seems to be a consensus that running tox without site packages is a better idea, in particular since many of our dependencies are now available as wheels and thus would install quickly provided one uses a recent pip/virtualenv.