[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.