Swig 3.0.2 (currently installed by Debian), install strongly typed
enumerator in the main namespace instead of in its own namespace.
This is fixed in latter versions of Swig.

* wrap/python/spot.py: If spot.op does not exists, populated it with all
operators from the spot namespace.
* wrap/python/tests/ltlsimple.py: Use operators from spot.op.