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.
.. _cwprops:
The property mecanism
---------------------
.. XXX CWProperty and co
Property API
~~~~~~~~~~~~
.. XXX feed me
Registering and using your own property
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. XXX feed me