Pyfields

* User-provided `nonable` status was wrongly overriden automatically when the field was attached to the class. Fixed [51](https://github.com/smarie/python-pyfields/issues/51)
* Fixed an issue with type validation when `typeguard` is used and a tuple of types is provided instead of a `Union`. Fixed [52](https://github.com/smarie/python-pyfields/issues/52)

See [documentation page](https://smarie.github.io/python-pyfields/changelog/) for details.

Added `pyproject.toml`

See [documentation page](https://smarie.github.io/python-pyfields/changelog/) for details.

Overall behaviour stabilized and compliance with `autoclass` to cover most use cases.

The only bug that has not yet been fixed is [12](https://github.com/smarie/python-pyfields/issues/12)

See [documentation page](https://smarie.github.io/python-pyfields/changelog/) for details.

**API**

- new helper methods `get_field`, `yield_fields`, `has_fields` and `get_fields` (new name of `collect_all_fields`) so that other libraries such as `autoclass` can easily access the various information. `fix_fields` removed. Fixed [48](https://github.com/smarie/python-pyfields/issues/48)

- New `ancestor_fields_first` option in all the `__init__` makers (`make_init` and `init_fields`). Fixed [50](https://github.com/smarie/python-pyfields/issues/50)

**Bugfixes**

- Bugfixes in all the `__init__` makers (`make_init` and `init_fields`):

- bugfix in case of inheritance with override: [49](https://github.com/smarie/python-pyfields/issues/49)

- the argument order used for fields initialization (inside the generated init method body) was sometimes incorrect. This would trigger a bug when one field was requiring another one to initialize.

- when the list of fields received by `InitDescriptor` was an empty tuple and not `None`, the constructor was not created properly

See [documentation page](https://smarie.github.io/python-pyfields/changelog/) for details.

- Fields can now be `nonable`, so as to bypass type and value validation when `None` is received. Fixed [44](https://github.com/smarie/python-pyfields/issues/44)

See [documentation page](https://smarie.github.io/python-pyfields/changelog/) for details.

- Now all type validation errors are `FieldTypeError`. Fixed [40](https://github.com/smarie/python-pyfields/issues/40).
- Fixed bug with python < 3.6 where fields were not automatically attached to their class when used from within a subclass first. Fixed [41](https://github.com/smarie/python-pyfields/issues/41)

See [documentation page](https://smarie.github.io/python-pyfields/changelog/) for details.