migrate IntoPyCallbackOutput to use IntoPyObject

[Icxolu]

This migrates IntoPyCallbackOutput blanket to use IntoPyObject.

[mejrs]

Thanks!

[mejrs]

IntoPyCallbackOutput is hidden, so I don't think this needs a changelog..unless it enables users to do things they couldn't do before (does it?). In which case we should document that instead.

[Icxolu]

Oh, I didn't notice that the whole module was hidden (maybe it should be moved into impl_). This should not enable any new things. The trait is used however as a public bound in PyModuleMethods::add_wrapped and PyCFunction constructors. Because the blanket impl changed there could be breakage for downstream types that do not (yet) implement IntoPyObject. The additional lifetime is less of a concern in this case I guess.

[davidhewitt]

I think I'd suggest the following:

• Just skip the CHANGELOG from this PR
• Two follow ups (ideally to complete before 0.23), let's open them as issues:
  • Let's move pyo3::callback module into the impl_ submodule
  • Let's move the function constructors to a new trait. They don't need all the generic complexity of IntoPyCallbackOutput, it would be probably good enough to have a simple trait which is implemented for both T: IntoPyObject and Result<T, E> where T: IntoPyObject, E: Into<PyErr>

[davidhewitt]

I think I'd suggest the following:

• Just skip the CHANGELOG from this PR
• Two follow ups (ideally to complete before 0.23), let's open them as issues:
  • Let's move pyo3::callback module into the impl_ submodule
  • Let's move the function constructors to a new trait. They don't need all the generic complexity of IntoPyCallbackOutput, it would be probably good enough to have a simple trait which is implemented for both T: IntoPyObject and Result<T, E> where T: IntoPyObject, E: Into<PyErr>