fix band_gaps_rigid.py example for Python 3
fix band_gaps_rigid.py example for Python 3

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