[sources] synchronize source asynchronously when started from the UI and redirect to the forthcoming import log, for a better user experience (there is still a lot more to do there though). Closes #10468967

[test] fix unittest_devctl on python3.4

[test] Make sure that newcube command test produces expected files

[server] remove some pre 3.10 compat Let's assume nobody will try upgrading an instance from pre-3.10 cubicweb to 3.23+. If they do, they're setting themselves up for a lot of pain anyway.