[fix] Python 2 / 3

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ERROR: test_func (ssa.simulation.generator_tests.RandomGraphTest)
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Traceback (most recent call last):
  File "/home/travis/build/vermiculus/ssa-tool/ssa/simulation/generator_tests.py", line 37, in test_func
    for c in ['yes', 'no', 'maybe']])
  File "/home/travis/build/vermiculus/ssa-tool/ssa/simulation/generator_tests.py", line 37, in <listcomp>
    for c in ['yes', 'no', 'maybe']])
AttributeError: 'map' object has no attribute 'count'