add PyInt constructor for all supported number types (i32, u32, i64, u64, isize, usize, f64)

[Trolldemorted]

Solves a part of #2221.

I deliberately did not implement support for PyLong_FromString because I don't feel familiar enough with the codebase to do this in my first PR.

I am aware that my ToPyInt trait will not work with PyLong_FromString, because PyLong_FromString takes more parameters than just a single value. I acknowledge that changing the trait to also accommodate PyLong_FromString will be a breaking change, but one could also add a dedicated constructor for that instead of squeezing it into new.

[Icxolu]

Thanks for working on this! In my opinion we should not introduce a new trait for this. This basically reimplements a subset of IntoPyObject. If we want to provide a generic constructor here, I think we should reuse the existing implementations and be generic over T: IntoPyObject<Target = PyInt, Error = Infallible>. I think that would cover all the (un)signed integer conversion. It would not cover f64 but I don't think that's a huge deal, it can easily be cast in Rust first and it is probably better to have this conversion explicit anyway.