This behavior wasn't documented and didn't fully work: it breaks
as soon as you try to save a model with a `None` value.

This allows using the latest annotation syntax supported by the type
checker regardless of the runtime Python version.

TODO: Review annotations after code changes made later.

The `Choices` constructor actually only accepts lists and tuples, not
arbitrary sequences. However, using `list` in the annotation opens
a can of worms related to type variance.

This allows using the latest annotation syntax supported by the type
checker regardless of the runtime Python version.

These annotations are sufficient to pass mypy inspection if mypy is
configured to allow unannotated functions.

This required a bit of refactoring to get the type of `STATUS` correct
for each suite.

There are two cases which I decided not to support in the type system:
- passing a list instead of a tuple when defining an option group
- `in` checks using a data type that doesn't match the choices

We also type check the unit tests and the unit tests now import
functionality from pytest.

I can't find a way to inform mypy of the actual types without
duplicating a lot of test code.

This preserves the type of the wrapped descriptor (usually a field).

Maybe this is overkill, as `DescriptorWrapper` seems to only be used
as part of the `FieldTracker` implementation and is not documented
and barely tested. But technically, it is public API.