[sources/native] fix potential NameError (closes #7364249)

[py3k] unicode → six.text_type

[devtools/testlib] Port CubicWebTC.view() to py3k testlib.TestCase.set_description's argument will eventually be printed out to sys.stdout/stderr. On python 2, this object takes bytes-like objects (eg, str) whereas python 3's takes unicode-like objects and handles the encoding on its own. Thus the 'str' type can used in both versions to eventually display the description on screen.