[CentOS] Use dnf instead of yum with Python 3

[KSerrania]

What does this PR do?

Use the dnf task instead of yum when we detect that a Python 3 interpreter is used on a target host.
Add CentOS 8 tests to the CI.
Add Python 2 / Python 3 tests to the CI.

Motivation

The yum Python module is not compatible with Python 3.
Should fix #298.

Additional Notes

CentOS / RHEL < 8 + Python 3 interpreter is an invalid setup (you can't use the dnf module either because the python3-dnf package doesn't exist on these versions of CentOS), so we fail early in that case, with a helpful error message.

[KSerrania]

I'm not 100% sure that we should fail hard when we detect Python 3 + CentOS < 8. On one side, it quickly gives a helpful message and avoids going through half of the role tasks only to find out that it doesn't work, but on the other side, there could be a case where someone backports dnf in their CentOS 7 box, in which case the role could succeed.

[mx-psi]

I'm not 100% sure that we should fail hard when we detect Python 3 + CentOS < 8. On one side, it quickly gives a helpful message and avoids going through half of the role tasks only to find out that it doesn't work, but on the other side, there could be a case where someone backports dnf in their CentOS 7 box, in which case the role could succeed.

We could add a flag to ignore the hard fail to deal with that edge case. I don't think it's a likely situation but it doesn't harm to have that flag I suppose.

[apigirl]

👍 docs look good