* wrap/python/spot.py: Python 3.5 reports some unexpected SystemError
messages when the stack of iterator(...(iterator(generator))) we build
for random LTL generation raises a StopIteration.  The messages are
attached either to delete_formula or delete_randltlgenerator, claiming
that these functions exit with an error; but I have checked that they
do not.  I've been unable to understand the cause of the issue.
Replacing the generator by an iterator at least fixes the problem in a
way that is transparent for our API.
* wrap/python/tests/randltl.ipynb: Adjust expected formulas.