experimental-inspect: slot trailing arguments are introspected without their None default #6364

Bug Description

CPython's slot wrappers let the caller omit a trailing argument and substitute None. Introspection emits those parameters as required, so the stub disagrees with the runtime and mypy.stubtest reports one error per method.

Affects __pow__, __rpow__ and __get__.

Steps to Reproduce

#[pyclass]
struct Number(u64);

#[pymethods]
impl Number {
    fn __pow__(&self, other: &Self, modulo: Option<&Self>) -> Self {
        Self(self.0.pow(other.0 as u32) % modulo.map_or(u64::MAX, |m| m.0))
    }
}

Build with experimental-inspect, then generate stubs with pyo3-introspection:

def __pow__(self, other: object, modulo: object, /) -> Number: ...   # generated

$ python -c "import slotrepro; print(slotrepro.Number.__pow__.__text_signature__)"
($self, value, mod=None, /)

$ python -m mypy.stubtest slotrepro
error: slotrepro.Number.__pow__ is inconsistent, runtime parameter "mod" has a default value but stub parameter does not
Stub:
def (slotrepro.Number, object, object) -> slotrepro.Number
Runtime:
def (self, value, mod=None, /)

Fix

I already worked on a fix here (#6363) but am unsure whether this implementation is really clean & good. Open for suggestions!