[py3.7,test] Use cubicweb-file >= 2.2.2 This includes changeset aa4a3a3d0c18 fixing import of os.errno.

[flake8] Fix errors in cubicweb/ext/test/unittest_rest.py

[test] Shorten prefix string in test_rql_role_with_unknown_vid With Python3.7, this test fails as the error message returned by rest_publish() is: <p>an error occurred while interpreting this rql directive: ObjectNotFound('toto')</p> while we previously assumed a comma after 'toto'. Fixing this by only shortening the prefix used in out.startswith().