check for all string types when calling isinstance() rather than just str

Turns out there were some subtle bugs in OVL after the python 3 port in python 2, due to passing in single channels to ClassifierData.triggers() which didn't make a list out of a single channel. That's because when checking if channel is a string, using isinstance(channel, str), it isn't a string, it's a unicode type. unicode doesn't exist as a type in python 3 but there's a simple way to support both in the future package.