Migrate _pyi_private_set/get_type to TypeVar default for broader type checker support

[huynguyengl99]

Hi team,

Python 3.13 has added the default parameter for TypeVar (which we can use via typing_extensions for older Python versions).

With this feature, we can remove the fields _pyi_private_set_type, _pyi_private_get_type and the mypy plugin functions related to those.

For example, instead of:

class IntegerField(Field[_ST, _GT]):
    _pyi_private_set_type: float | int | str | Combinable
    _pyi_private_get_type: int
    _pyi_lookup_exact_type: str | int

We can define:

# **set** value type
_ST_Int = TypeVar("_ST_Int", contravariant=True, default=float | int | str | Combinable)
# **get** return type
_GT_Int = TypeVar("_GT_Int", covariant=True, default=int)

class IntegerField(models.Field[_ST_Int, _GT_Int]):
    _pyi_lookup_exact_type: str | int

By using this approach, we can eliminate the mypy plugin related to __pyi_private_set/get_type, allowing us to support other type checkers beyond mypy as well.

With this change, this code will work for all static type checkers:

my_num = IntegerField()

Rather than requiring:

my_num = IntegerField[float | int | str | Combinable, int]()

How do you think about this approach? I'm willing to create a PR for this improvement.

[UnknownPlatypus]

There is work in progress in #2590 to implement this. Feel free to have a look at the PR 🙌

[huynguyengl99]

Thank you, will take a look on that and close this one.