poisson_parallel_interactive.py: fix save_stats() for Python 3
poisson_parallel_interactive.py: fix save_stats() for Python 3

rc/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,sfepy/sfepy,rc/sfepy,vlukes/sfepy,vlukes/sfepy,rc/sfepy,BubuLK/sfepy,BubuLK/sfepy