SchemaDictProperty forces ascii key

[dannyroberts]

https://github.com/benoitc/couchdbkit/commit/038ee178393f13c9a5c45548442d53c41ab16bca

It seems like instead of forcing to a str it should do a check such as

assert isinstance(index, basestring)

or explicitly converting ints if that was the purpose. Alternatively, we could change str to unicode and call it a day.

[benoitc]

i am adding ti to the list of changes for the python 3 release for now. str/unicode/bytes will be seriously reworked at that time. I think having an assert may be good at that point anyway.