Support slices as fields

[victora-openai]

Is your feature request related to a problem? Please describe.
It would be useful to support serialization and parsing of slice objects.

For example,

@dataclass
class Foo(DataClassMessagePackMixin):
    my_slice: slice
    slice_dict: dict[slice, int]  # Legal in Python 3.12

f = Foo(slice(0, 5), {slice(3,6,2): 4})
s = f.to_msgpack()
Foo.from_msgpack(s)

Describe the solution you'd like
Slices should be natively serializable and parsable. The snippet above should run.

Describe alternatives you've considered
It's possible to hand write a slice wrapper class. This is what I've been doing but as slice has become hashable in Python 3.12, it would be nice to not have to have a non-trivial wrapping and unwrapping layer.

Additional context
This could be a good first issue as slices can basically be represented as tuples.

[Fatal1ty]

Sounds good to me. A slice object can be represented as a list of [start, stop, step] values. Assuming, that these values can be either integer or None, this would be trivial to implement.

However, I will note that neither a list nor a tuple can be used as dictionary keys, so it cannot be a key in MessagePack format.