First of all, thank you for this project! It's been invaluable in a few of my libraries.

Is your feature request related to a problem? Please describe.

Per #1779 and similar issues, datamodel-codegen currently produces unified oneOf types using RootModel as a base class, e.g.:

class Foo(RootModel[Union[Bar, Baz]]):
    root: Union[Bar, Baz]

This is semantically correct and supported by Pydantic. However, as noted in https://github.com/pydantic/pydantic/discussions/7418, some typecheckers (like mypy) struggle with this format.

For Python 3.8+, Pydantic recommends this form instead:

Foo = RootModel[Union[Bar, Baz]]

...which mypy does understand, and is able to infer Foo(...)'s params/kwargs for.

Describe the solution you'd like

When --target-python-version is 3.8 or newer, datamodel-codegen should emit (or be able to be configured to emit) the RootModel[...] annotation format, rather than the inheritance format.

Describe alternatives you've considered

I could just bushwack around this in my codebases by ignoring the mypy errors. But it'd be nice to have a generalized fix upstream here 😅

Additional context

N/A