feat: make ErrorType enums compare as strings (#269)

The DBusError exception stores the error type as string. This makes the
exception not directly compare to the ErrorType members (for example
DBusError(ErrorType.FAILED, "").type != ErrorType.FAILED). This makes
ErrorType also a string to make this comparision work.

src/dbus_fast/constants.py

@@ -91,7 +91,7 @@ class ArgDirection(Enum):
     OUT = "out"
 
 
-class ErrorType(Enum):
+class ErrorType(str, Enum):
     """An enum for the type of an error for a message reply.
 
     :seealso: http://man7.org/linux/man-pages/man3/sd-bus-errors.3.html