updated OS versions for the test suite

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 / python-redfish

updated OS versions for the test suite #103