Fix symposion.schedule.timetable method.
Fix symposion.schedule.timetable method.

  - Use global next() method:
    http://diveinto.org/python3/porting-code-to-python-3-with-2to3.html#next
  - Use `zip_longest` method instead of `izip_longest`:
    https://stackoverflow.com/a/5850582/113527

pydata/conf_site,pydata/conf_site,pydata/conf_site