[test] Replace logilab-common's with_tempdir by tempfile.TemporaryDirectory
logilab-common's with_tempdir decorator, which is considered harmful (see
https://www.logilab.org/ticket/8267966).
Rely on backports.tempfile to provide TemporaryDirectory on Python 2.
Re-export it in testlib module for convenience.
3.34 (UNRELEASED)
=================
New features
------------
* Various bits of a CubicWeb application configuration can be now be
overridden through environments variables matching configuration option
names prefixed by ``CW_`` (for instance ``CW_BASE_URL``).
* Cubes are now standard Python packages named as ``cubicweb_<cubename>``.
They are not anymore installed in ``<prefix>/share/cubicweb/cubes``. Their
discovery by CubicWeb is handled by a new setuptools entry point
``cubicweb.cubes``. A backward compatibility layer is kept for "legacy"
cubes.