[test] use unittest.mock instead of mock library Now that we use Python 3 only, this is possible.

Drop python2 support This mostly consists in removing the dependency on "six" and updating the code to use only Python3 idioms. Notice that we previously used TemporaryDirectory from cubicweb.devtools.testlib for compatibility with Python2. We now directly import it from tempfile.

Merge with 3.26

Remove unused imports in cubicweb.pyramid.config