python: improve bdd_dict bindings
Fixes #372. * python/spot/impl.i: Refactor the handling of exceptions using a Lippincott function. Map out_of_range to IndexError. Add PyObject* version for bdd_dict's register and unregister functions so we can use Python objects as well. * tests/python/bdddict.py: New file. * tests/Makefile.am: Add it. * NEWS: Mention the changes.