cubicweb #5447161 add configuration for tox [validation pending]

With the aim of exposing a somehow "standard" (i.e. well known by the community) way to develop CubicWeb, I propose to add support for running test suites with tox.

For this we need to add a tox.ini file with possibly different test environments (for instance, the different CubicWeb sub-packages like web, hooks, etc.).

Tox uses virtualenv to run tests (and check distribution and installation of the project), these are usually filled by specifying the dependencies of the project either directly in the tox.ini file or within requirements files. The latter approach appears to be quite wide-spread, so I'd suggest to go this way. One thing that needs to be discussed is how these requirements files would be organized. We could have a main requirements.txt (or several following dependencies of Debian packages for instance) and one (or several) requirements-test.txt that would list dependencies for tests (cubes for instance).

Comments welcome.

done in3.21.0
load left0.000
closed by#99bdd4bddd77 Add tox.ini file
patch[tox] Update doc testenv to account for documentation restructuring [applied]Add tox.ini file [applied]Add requirements.txt files in test directories [applied]