Variable: Implement proper hash

The old implementation allowed Variable instances which were equal to
have different hash.

Orange/data/tests/test_variable.py

@@ -443,6 +443,8 @@ def test_make_proxy_disc(self):
         self.assertEqual(abc, abc1)
         self.assertEqual(abc, abc2)
         self.assertEqual(abc1, abc2)
+        self.assertEqual(hash(abc), hash(abc1))
+        self.assertEqual(hash(abc1), hash(abc2))
 
         abcx = DiscreteVariable("abc", values="abc", ordered=True)
         self.assertNotEqual(abc, abcx)
@@ -469,6 +471,8 @@ def test_make_proxy_cont(self):
         self.assertEqual(abc, abc1)
         self.assertEqual(abc, abc2)
         self.assertEqual(abc1, abc2)
+        self.assertEqual(hash(abc), hash(abc1))
+        self.assertEqual(hash(abc1), hash(abc2))
 
     def test_proxy_has_separate_colors(self):
         abc = ContinuousVariable("abc")

Orange/data/variable.py

@@ -311,7 +311,10 @@ def __eq__(self, other):
         return hasattr(other, "master") and self.master is other.master
 
     def __hash__(self):
-        return super().__hash__()
+        if self.master is not self:
+            return hash(self.master)
+        else:
+            return super().__hash__()
 
     @classmethod
     def make(cls, name):