metadata: Rename _Delegator to _DelegatorMixin

Make it clearer that this is not part of the main inheritance path.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tuf/api/metadata.py

@@ -634,7 +634,7 @@ def to_dict(self) -> Dict[str, Any]:
         }
 
 
-class _Delegator(metaclass=abc.ABCMeta):
+class _DelegatorMixin(metaclass=abc.ABCMeta):
     """Class that implements verify_delegate() for Root and Targets"""
 
     @abc.abstractmethod
@@ -710,7 +710,7 @@ def verify_delegate(
             )
 
 
-class Root(Signed, _Delegator):
+class Root(Signed, _DelegatorMixin):
     """A container for the signed part of root metadata.
 
     Parameters listed below are also instance attributes.
@@ -1784,7 +1784,7 @@ def get_prefixed_paths(self) -> List[str]:
         return paths
 
 
-class Targets(Signed, _Delegator):
+class Targets(Signed, _DelegatorMixin):
     """A container for the signed part of targets metadata.
 
     Targets contains verifying information about target files and also