No need for unicode

Missed it while reviewing !2, but as we use `from builtins import str`, there is no need to use `unicode`.

flatisfy/config.py

@@ -95,7 +95,7 @@ def validate_config(config):
         # message in the log output.
         # pylint: disable=locally-disabled,line-too-long
         assert "type" in config["constraints"]
-        assert isinstance(config["constraints"]["type"], (str, unicode))
+        assert isinstance(config["constraints"]["type"], str)
         assert config["constraints"]["type"].upper() in ["RENT",
                                                          "SALE", "SHARING"]