Support ClassVar fields

[dhrosa]

See https://docs.python.org/3.12/library/dataclasses.html#class-variables

This will be a bit tricky as MicroPython does not have a typing module. We could do something like having an extra compat submodule that provides extensions to the standard dataclasses API, where we can implement our own ClassVar.

[smurfix]

MicroPython does not have a typing module

Easy enough to add a stub:

M-o-a-T/micropython-lib@9ca707b0b

[dhrosa]

Good point on adding stubs!

Though even if we had a stub for ClassVar, I think we have problems.

We can't see type annotations in MicroPython, so the only way to see that a field has a type of ClassVar would be to inspect the type of the field's value, which would require declaring the field something like this:

@dataclass
class C:
    class_field = ClassVar[int](0)

Reading more on https://docs.python.org/3.12/library/typing.html#typing.ClassVar

ClassVar is not a class itself, and should not be used with isinstance() or issubclass(). ClassVar does not change Python runtime behavior,

ClassVar does not support actually being instantiated like a class, or having isinstance() used on it. It is possible to have udataclass's fork of ClassVar support this, but it would be incompatible with stdlib's ClassVar.