Support yaml 1.2 files

[domna]

We currently use pyyaml, which only supports yaml 1.1 and it's in question if and when yaml 1.2 support will come, see yaml/pyyaml#116.

Using yaml 1.1 is problematic and unintuitive in some cases, e.g., yes and no get converted to boolean which can be problematic for some cases, see #12 (review) for an example.

We should see if there are better drop-in replacements for pyyaml which support yaml 1.2, too.

[domna]

Switchting to https://pypi.org/project/ruamel.yaml/ could be a solution