Add relationship for end_transaction

We had a relationship for UserVersion.transaction
But no equivalent relationship for UserVersion.end_transaction

This commit adds that and makes it consistent.
end_transaction is only present for validity strategy - so, add that
only in validity cases

sqlalchemy_continuum/model_builder.py

@@ -169,6 +169,19 @@ def build_transaction_relationship(self, tx_class):
                 foreign_keys=[transaction_column],
             )
 
+        if self.manager.option(self.model, 'strategy') == 'validity':
+            end_transaction_column = getattr(
+                self.version_class,
+                tx_column_name(self.model)
+            )
+
+            if not hasattr(self.version_class, 'end_transaction'):
+                self.version_class.end_transaction = sa.orm.relationship(
+                    tx_class,
+                    primaryjoin=tx_class.id == end_transaction_column,
+                    foreign_keys=[end_transaction_column],
+                )
+
     def base_classes(self):
         """
         Returns all base classes for history model.