Support for `pyclass(new = "from_fields")`

[dmelcer9]

The pyclass macro has an option new="from_fields" that generates a simple default constructor. It seems that pyo3-stub-gen does not pick this up, requiring a manual boilerplate implementation in order to correctly generate type stubs.

Input:

#[gen_stub_pyclass]
#[pyclass(new = "from_fields")]
pub struct Foo {
    pub bar: bool,
}

Expected type stub:

@typing.final
class Foo:
    def __new__(cls, bar: builtins.bool) -> Foo: ...

Actual:

@typing.final
class Foo:
    ...

Environment:

rustc 1.94.0 (4a4ef493e 2026-03-02)
pyo3 = "0.28.2"
pyo3-stub-gen = "0.19.0"