Part of #467.
* tests/python/bdddict.py, tests/python/ltl2tgba.py,
tests/python/ltlparse.py, tests/python/ltlsimple.py,
tests/python/sccinfo.py, tests/python/simstate.py,
tests/python/split.py, tests/python/tra2tba.py: Adjust to
deal with a non-refcounted Python implementation.
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.