Yamlable

- Objects (subclasses of `YamlAble` or `YamlObject2`) can now be loaded from both mappings, sequences and scalars. Users may override the default behaviour by overriding `__from_yaml_list__` and `__from_yaml_scalar__`. Codecs (subclasses of `YamlCodec`) can also support this feature, through the `from_yaml_list` and `from_yaml_scalar` methods (not implemented by default). Fixes [12](https://github.com/smarie/python-yamlable/issues/12)
- Now using `mkdocs-gallery` for documentation examples. Fixed [14](https://github.com/smarie/python-yamlable/issues/14)
- `yaml_info` can not be used on subclasses of `YamlObject2` anymore as it could lead to an unexpected behaviour. Fixes [15](https://github.com/smarie/python-yamlable/issues/15)

- Most type hints have been fixed, in particular for `yaml_info`. Fixes [11](https://github.com/smarie/python-yamlable/issues/11).
- CI/CD pipeline migrated to Github Actions. Added badges using `genbadge`

- packaging improvements: set the "universal wheel" flag to 1, and cleaned up the `setup.py`. In particular removed dependency to `six` for setup and added `py.typed` file, as well as set the `zip_safe` flag to False. Removed tests folder from package. Fixes [10](https://github.com/smarie/python-yamlable/issues/10)

Added `pyproject.toml`.

Added `__version__` pkg-level attribute.

* this version has been out and stable for long enough to be considered a good 1.0.0

* PyYaml 5.1 added some breaking changes. We now comply with them while still supporting the old versions. Fixed [8](https://github.com/smarie/python-yamlable/issues/8).