Remove py2 compat from api.serialization package

tuf.api is not designed for Python 2 compatibility. This commit
removes the following stray compatibility constructs in its
serialization subpackage:

- '__metaclass__ = abc.ABCMeta'
- six.raise_from

Author: Lukas Puehringer

tuf/api/serialization/__init__.py

@@ -26,29 +26,26 @@ class DeserializationError(Exception):
     """Error during deserialization. """
 
 
-class MetadataDeserializer():
+class MetadataDeserializer(metaclass=abc.ABCMeta):
     """Abstract base class for deserialization of Metadata objects. """
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def deserialize(self, raw_data: bytes) -> "Metadata":
         """Deserialize passed bytes to Metadata object. """
         raise NotImplementedError
 
 
-class MetadataSerializer():
+class MetadataSerializer(metaclass=abc.ABCMeta):
     """Abstract base class for serialization of Metadata objects. """
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def serialize(self, metadata_obj: "Metadata") -> bytes:
         """Serialize passed Metadata object to bytes. """
         raise NotImplementedError
 
 
-class SignedSerializer():
+class SignedSerializer(metaclass=abc.ABCMeta):
     """Abstract base class for serialization of Signed objects. """
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def serialize(self, signed_obj: "Signed") -> bytes:

tuf/api/serialization/json.py

@@ -10,7 +10,6 @@
 
 """
 import json
-import six
 
 from securesystemslib.formats import encode_canonical
 
@@ -36,7 +35,7 @@ def deserialize(self, raw_data: bytes) -> Metadata:
             metadata_obj = Metadata.from_dict(json_dict)
 
         except Exception as e: # pylint: disable=broad-except
-            six.raise_from(DeserializationError, e)
+            raise DeserializationError from e
 
         return metadata_obj
 
@@ -63,7 +62,7 @@ def serialize(self, metadata_obj: Metadata) -> bytes:
                                     sort_keys=True).encode("utf-8")
 
         except Exception as e: # pylint: disable=broad-except
-            six.raise_from(SerializationError, e)
+            raise SerializationError from e
 
         return json_bytes
 
@@ -78,6 +77,6 @@ def serialize(self, signed_obj: Signed) -> bytes:
             canonical_bytes = encode_canonical(signed_dict).encode("utf-8")
 
         except Exception as e: # pylint: disable=broad-except
-            six.raise_from(SerializationError, e)
+            raise SerializationError from e
 
         return canonical_bytes