Update code to Python 3.5

See https://docs.python.org/3.0/whatsnew/3.0.html and https://docs.python.org/3.5/whatsnew/3.5.html

• replace from module import class by from .module import class
• replace __unicode__ by __str__
• remove u before string literals
• replace json.dumps(schedule, ensure_ascii=False, encoding='utf-8') by json.dumps(schedule, ensure_ascii=False)