version.py: Implement _object_links.

Author: schwehr

pystac/extensions/version.py

@@ -88,7 +88,7 @@ def from_item(cls, an_item):
 
     @classmethod
     def _object_links(cls):
-        return []  # TODO(schwehr): What should this return?
+        return [LATEST_VERSION, PREDECESSOR_VERSION, SUCCESSOR_VERSION]
 
 
 class VersionCollectionExt(base.CollectionExtension):
@@ -150,7 +150,7 @@ def from_collection(cls, a_collection):
 
     @classmethod
     def _object_links(cls):
-        return []  # TODO(schwehr): What should this return?
+        return [LATEST_VERSION, PREDECESSOR_VERSION, SUCCESSOR_VERSION]
 
     def apply(self, version, deprecated=None, latest=None, predecessor=None, successor=None):
         self.version = version