Fix blank lines in tuf.api as per styleguide

https://github.com/google/styleguide/blob/gh-pages/pyguide.md#35-blank-lines

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

tuf/api/metadata.py

@@ -266,7 +266,6 @@ def verify(self, key: Mapping[str, Any],
             signed_serializer.serialize(self.signed))
 
 
-
 class Signed:
     """A base class for the signed part of TUF metadata.
 
@@ -281,12 +280,10 @@ class Signed:
             metadata format adheres to.
         expires: The metadata expiration datetime object.
 
-
     """
     # NOTE: Signed is a stupid name, because this might not be signed yet, but
     # we keep it to match spec terminology (I often refer to this as "payload",
     # or "inner metadata")
-
     def __init__(
             self, _type: str, version: int, spec_version: str,
             expires: datetime) -> None:
@@ -333,13 +330,11 @@ def _common_fields_to_dict(self) -> Dict[str, Any]:
             'expires': self.expires.isoformat() + 'Z'
         }
 
-
     # Modification.
     def bump_expiration(self, delta: timedelta = timedelta(days=1)) -> None:
         """Increments the expires attribute by the passed timedelta. """
         self.expires += delta
 
-
     def bump_version(self) -> None:
         """Increments the metadata version number by 1."""
         self.version += 1
@@ -420,7 +415,6 @@ def add_key(self, role: str, keyid: str,
             self.roles[role]['keyids'].append(keyid)
             self.keys[keyid] = key_metadata
 
-
     # Remove key for a role.
     def remove_key(self, role: str, keyid: str) -> None:
         """Removes key for 'role' and updates the key store. """
@@ -433,8 +427,6 @@ def remove_key(self, role: str, keyid: str) -> None:
             del self.keys[keyid]
 
 
-
-
 class Timestamp(Signed):
     """A container for the signed part of timestamp metadata.
 

tuf/api/serialization/__init__.py

@@ -16,10 +16,12 @@
 """
 import abc
 
+
 # TODO: Should these be in tuf.exceptions or inherit from tuf.exceptions.Error?
 class SerializationError(Exception):
     """Error during serialization. """
 
+
 class DeserializationError(Exception):
     """Error during deserialization. """