bpo-33441: Make the sigset_t converter available in other modules. (GH-6720)

bpo-33441: Make the sigset_t converter available in other modules. (GH-6720)

* Expose the sigset_t converter via private API _Py_Sigset_Converter().
* Use Argument Clinic for parsing sigset_t in signalmodule.c.
* Raise ValueError instead OverflowError for integers out of
  the C long range.

Based on patch by Pablo Galindo Salgado.