gh-109598: make PyComplex_RealAsDouble/ImagAsDouble use __complex__ (GH-109647)

`PyComplex_RealAsDouble()`/`PyComplex_ImagAsDouble` now try to convert
an object to a `complex` instance using its `__complex__()` method
before falling back to the ``__float__()`` method.

PyComplex_ImagAsDouble() also will not silently return 0.0 for
non-complex types anymore.  Instead we try to call PyFloat_AsDouble()
and return 0.0 only if this call is successful.