* use yaml 1.2 everywhere
This commit:
* gets rid of pyyaml
* uses ruamel everywhere
* tries to unite toml and yaml
However, this will require a few more changes that will come soonish.
Eg: Current `parse_yaml` and `parse_toml` behavior could be error-prone,
so, it's better to introduce `tree` inside these serialization methods
(which I will try to tackle next).
Another is, yaml load can be slow for json. We could speed it up just by
using `json.load(s)` here (also in upcoming patch).
* Fix tests