various: Move _sentinel from utils.py to _yaml.py

The `_sentinel` in `utils.py` was used only for yaml node tracking.
As such, simplify matters by removing it from `utils.py` and move it
to `_yaml.py` which means that we no longer need to try and avoid a
circular import issue by means of runtime importing.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>