Layered-config-tree

- Bugfix: Raise a MissingLayerError if a requested value exists but not at the requested layer.
- Get nested values from a single 'get' or 'get_tree' call
- Move tree.get_from_layer() logic into tree.get() and add deprecation warning.
- Utilize centralized build tools

- Raise an error if YAML contains duplicate keys within the same level

- Better handle dunder-style keys

- Bugfix: failing mypy

- Drop support for Python 3.9

- Add getter methods