Add pytest-subtests to dev requirements This pytest plugin adds support for unittest's TestCase.subTest (for Python >= 3.4). Since we we now only support python3 and since we use a fair amount of these, let's benefit of better reporting by using this plugin.

[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