using subclass, args on new, and overriding __init__ fails

This issue is about sub classing with overridden init ; This bug prevents it. Details below.

🐛 Bug Reports

When reporting a bug, please provide the following information. If this is not a bug report you can just discard this template.

🌍 Environment

• Your operating system and version: linux, ubuntu focal (20.04)
• Your python version: 3.8
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv? ubuntu package (apt-get). using a virtualenv for testing.
• Your Rust version (rustc --version): rustc 1.54.0-nightly (b663c0f4f 2021-05-29)
• Your PyO3 version: master 8bf3adee3a0da3ce1cf19ed5e20bd4759edfa3ae (v0.8.0-1694-g8bf3adee3) (Merge pull request #1641 from 1tgr/for-each)
• Have you tried using latest PyO3 main (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?yes.

💥 Reproducing

I have a branch with the augmented test (examples/pyo3-pytests/src/subclassing.rs & examples/pyo3-pytests/tests/test_subclassing.py), at https://github.com/alonblade/pyo3/tree/add-test-for-inheritance-with-init ; Here is the change:

#[pyclass(subclass)]
pub struct SubclassableWithParameter {}

#[pymethods]
impl SubclassableWithParameter {
    #[new]
    fn new(foo: bool) -> Self {
        SubclassableWithParameter {}
    }
}

class SubclassWithExtraInitArguments(SubclassableWithParameter):
    def __init__(self, bar):
        print("before super init")
        super().__init__(foo=bar * 2)


def test_subclass_with_init():
    s = SubclassWithExtraInitArguments(10)

And the output

tests/test_subclassing.py .F                                                                                                                                                                                  [100%]

===================================================================================================== FAILURES ======================================================================================================
______________________________________________________________________________________________ test_subclass_with_init ______________________________________________________________________________________________

    def test_subclass_with_init():
>       s = SubclassWithExtraInitArguments(10)
E       TypeError: argument 'foo': 'int' object cannot be converted to 'PyBool'

tests/test_subclassing.py:27: TypeError

Start with the reproducer in examples/pyo3-pytests/src/subclassing.rs and examples/pyo3-pytests/tests/test_subclassing.py, then run test_subclass_with_init to confirm the TypeError. Trace how the subclass calls super().init(foo=bar * 2); done means this test passes without the reported conversion failure.