Add support to Python3

[vitorleal]

StandardError does not exist any more in Python3 (https://docs.python.org/3/library/exceptions.html). The suggestion of migrating Python2 to Python3 and supporting both versions is to change to Exception.

reference

[jberends]

this project seems a bit dormant, being 5 years ago with the last update. I guess this service is stable and making its money, but innovation is limited.