Add CentOS 10, Debian 11 and openSUSE 15 tests

[evgeni]

• don't set max-parallel 1, run all the tests in parallel
• fail-fast: false - run all tests even if some fail
• add EL10 testing
• use native unittest, unittest2 was a backport for pre 2.7 python
• stop using deprecated imp -- import and ImportError is enough
• properly skip DNF related tests when DNF is not available
• Add Debian 11 tests

[evgeni]

EL6 is failing, but that will be removed in #161

[ekohl]

I see some good cleanups here. I still think this repository has poor abstraction. Ideally we'd have some base class and then for each "backend" an implementation. Some changes I made in the past moved in this direction, but it's not there yet.

[ekohl]

If you change it to this, then this part becomes redundant:

katello-host-tools/src/katello/enabled_report.py

Lines 5 to 6 in ecd9b1a

	if YUM:
	import yum

[evgeni]

how so? it would then need to from katello.constants import yum instead…

[ekohl]

Oh yes, Ruby has created some brain damage where I assumed it was present in a global namespace.