[devtools] Make dependency on backports.tempfile (Python2) optional The package may not be available in all systems (e.g. no Debian package exist at the moment), and we should not crash with ImportError when importing testlib from client code. Follow up on a6dc650bc230 where the dependency was introduced.

[sources] Drop source support_entity / support_relation source API this was used in the pre-datafeed area, it's not worth it anymore. Its only valid usage was in authentication to detect if the source was supporting CWUser, hence we now call it systematically and catch NotImplementedError.

[dataimport] Add a new NullStore This is useful to do some import tests where you only want to test steps prior to the database insertion, as well as "dry-run" commands. Use it as a documenting base class.

[devtools] Properly reraise the exception with previous implementation, the original exception wasn't visible in python 2.

[pkg] Do not install sql scripts in <prefix>/share/cubicweb/schemas These are already site-packages along with cubicweb package.

[devtools/qunit] Retain firefox process stdout to eventually print it The bare RuntimeError that gets emitted upon timeout does not help much to diagnose the problem. Let's see if we can get more information from stdout by not dropping it.

[devtools/qunit] Turn FirefoxHelper into a context manager So that stop() method always gets called. Also ensure temporary directory created on __init__ gets deleted as well upon exit.

[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.

[test] Use `with open` syntax in cwconfig "prefix" tests

[test] Use stdlib unittest.main() in qunit