Add support for SetType

Define a new schematics type to be used with cc_dynamodb3: SetType. This type is derived from the schematics ListType. It forces its Python (native) representation to be a Python set.
Its purpose is for DynamoDB Number Sets and String Sets. Boto3 will do the conversion of Python sets containing Numbers (int or Decimal) to DynamoDB Number Sets and Python sets containing strings to DynamoDB String Sets.