* 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
Currently `RepoTree.open` will throw FileNotFoundError if you have a
file that is tracked by dvc, but which has no md5 or a cache for which
is missing, even though you have the file itself in your workspace.
This corresponds very nicely to the `--no-commit` or `--no-exec`
scenario, where you might not want to commit your artifacts to cache
just yet.
Fixes#3974
Saugat Pachhai
Ruslan Kuprieiev
Casper da Costa-Luis
Ruslan Kuprieiev