Multirate stable timestep finder reports different timesteps on py2 versus py3

@inducer, not sure where the problem lies.

$ python2 multirate-step-matrix.py 
speed factor: 10 - step ratio: 7 - method: Fq - order: 3
('stable imaginary timestep:', 5.0653839111328125)
('stable neg real timestep:', 0.5454483032226562)

$ python3 multirate-step-matrix.py 
speed factor: 10 - step ratio: 7 - method: Fq - order: 3
stable imaginary timestep: 0.04102325439453125
stable neg real timestep: 3.8001632690429688