make Batches sortable [dunder_gt]

Author: hjwp

model.py

@@ -25,6 +25,9 @@ def __init__(self, ref: str, sku: str, qty: int, eta: Optional[date]):
         self._purchased_quantity = qty
         self._allocations = set()  # type: Set[OrderLine]
 
+    def __repr__(self):
+        return f"<Batch {self.reference}>"
+
     def __eq__(self, other):
         if not isinstance(other, Batch):
             return False
@@ -33,6 +36,13 @@ def __eq__(self, other):
     def __hash__(self):
         return hash(self.reference)
 
+    def __gt__(self, other):
+        if self.eta is None:
+            return False
+        if other.eta is None:
+            return True
+        return self.eta > other.eta
+
     def allocate(self, line: OrderLine):
         if self.can_allocate(line):
             self._allocations.add(line)