Accept key_properties as tuple

[kcolton]

This check is a little bothersome.

Out of habit, will use tuples over lists if i'm working with something immutable.

Hit Exception("key_properties must be a string or list of strings") while making first python tap, just out of the habit of using tuples for definition like this.

singer.write_schema('looker_data', schema, key_properties=('users.joined_week', 'users.channel'))

(Hint at the WIP tap :bowtie:)

Whether key_properties should accept strings at all, or if this is the best way of checking, are questions for another day. Just wanted the smallest diff. All tests pass, and didn't see any other checks this could mess with at a glance.