[deps] Upgrade python-six to Debian 10 version as minimum

2 jobs for topic/default/py-six-deb10 in 9 seconds (queued for 30 seconds)
Name Stage Failure
pytest Tests
$ python setup.py install
Traceback (most recent call last):
File "/builds/cubicweb/cubes/slickgrid/setup.py", line 35, in <module>
exec(f.read(), __pkginfo__)
File "<string>", line 13, in <module>
NameError: name 'dirname' is not defined
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: command terminated with exit code 1