style: fix flake8 issues

Author: yt-ms

src/structurizr/model/implied_relationship_strategies.py

@@ -56,7 +56,7 @@ def create_implied_relationships_unless_any_exist(relationship: Relationship):
 
 def create_implied_relationships_unless_same_exists(relationship: Relationship):
     """
-    Create implied relationships unless there is a existing one with the same description.
+    Create implied relationships unless there is one with the same description.
 
     This strategy creates implied relationships between all valid combinations of the
     parent elements, unless any relationship already exists between them which has the

src/structurizr/model/relationship.py

@@ -105,13 +105,15 @@ def __init__(
 
     @property
     def source_id(self) -> str:
+        """Return the ID of the source element of this relationship."""
         if self.source is not None:
             return self.source.id
 
         return self._source_id
 
     @property
     def destination_id(self) -> str:
+        """Return the ID of the destination element of this relationship."""
         if self.destination is not None:
             return self.destination.id
 
@@ -128,7 +130,7 @@ def interaction_style(self) -> str:
 
     @classmethod
     def hydrate(cls, relationship_io: RelationshipIO) -> "Relationship":
-        """"""
+        """Hydrate a new instance of Relationship from its IO."""
         return cls(
             id=relationship_io.id,
             tags=relationship_io.tags,