Bugfix: Enum was ignored when used in combination with ConstantStr

[florianfischer91]

Let's consider the following situation

class Status(ConstantStr, enum.Enum):
    ON  = "on"
    OFF = "off"

 class Robot(Predicate):
     name: str
     status: Status

Because of a wrong priority (order) when trying to infer (create) the correct Field, Robot.status was handled as a ConstantField instead of a custom Field which restricts the allowed values. The result was that you could create a Robot-fact with status f.i. "standby"

This PR fixes that problem by first handling the case if the type of Status is an Enum and handle the case for a ConstantStr afterwards