Work around weird Python 3.5 generator/iterator interaction
* 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.