updated OS versions for the test suite

[SylvanBrocard]

Updated OS and python versions for the tests:

• Centos: same version, python 2 -> 3
• Ubuntu: 16.04 -> 20.04, python 2 -> 3
• Debian: 8 -> 11, python 2 -> 3
• Fedora (source): 25 -> 35, python 2 -> 3 for source
• Fedora (pip): 25 -> 35

Current pytest status: 2 failed, 3 passed, 113 warnings
First failure message:

Testing : rf-debian-11-src-p3
Traceback (most recent call last):
File "/usr/bin/redfish-client", line 28, in <module>
import redfish
ModuleNotFoundError: No module named 'redfish'

[bcornec]

Thanks for this work !