[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.

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missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)