yaml: change dependency from pyyaml to ruamel.yaml

[albestro]

Fix #272

This PR starts as a reproducer of the #272 (with a trivial test) and it becomes a tentative in fixing it.

• Fix ruamel.yaml dump function usage

[albestro]

We can bikeshed on the load change, but the real point is how to refactor in order to comply with the different dump API, which does not return strings (there is a simple workaround with StringIO, but I think it might be better to change a bit the paradigm).

Before investing time in a proper change, I quickly addressed the problem to give it a try and check if everything worked. I've not checked that scientifically (yet), but I was able to build a uenv image with this version.

If and when we decide to proceed on this path, I'm going to properly take care of the changes needed.