Fix test for objective offset (#269)

* ensure float is returned

* update RCs

* try with explicit conversion

* fix another instance of the bug

CHANGELOG.rst

@@ -3,6 +3,10 @@
 Next Release
 -----
 
+1.8.3
+-----
+* fix the objective offset test for compatibility with Debian sid
+
 1.8.2
 -----
 * fix the feasibility check in the hybrid solver

src/optlang/tests/abstract_test_cases.py

@@ -823,7 +823,11 @@ def test_objective_expression_includes_constant(self):
         objective = self.model.objective
         self.model.objective = self.interface.Objective(objective.expression + 3)
         self.model.update()
-        self.assertEqual((self.model.objective.expression - (objective.expression + 3.)).expand(), 0.)
+        self.assertEqual(
+            float((self.model.objective.expression -
+                  (objective.expression + 3.)).expand().evalf()),
+            0.
+        )
 
     def test_is_integer(self):
         model = self.model

src/optlang/tests/test_hybrid_interface.py

@@ -324,7 +324,11 @@ def test_constraint_set_problem_to_None_caches_the_latest_expression_from_solver
             self.assertEqual(constraint.name, "test")
             self.assertEqual(constraint.lb, -100)
             self.assertEqual(constraint.ub, None)
-            self.assertEqual((constraint.expression - (0.4 * y + 0.3 * x + 77.0 * z)).expand(), 0.0)
+            self.assertEqual(
+                float((constraint.expression -
+                       (0.4 * y + 0.3 * x + 77.0 * z)).expand().evalf()),
+                0.0
+            )
 
         def test_change_of_objective_is_reflected_in_low_level_solver(self):
             x = Variable('x', lb=-83.3, ub=1324422.)