pytest.raises: checking for exception error message fixed

- str(e) did work in previous pytest version
- but the correct usage is str(e.value)
- in newer pytest version the string for the e object is changed so
  the tests did not work any more

lbmpy_tests/test_lbstep.py

@@ -92,7 +92,7 @@ def test_advanced_initialization():
     shear_flow_scenario = create_fully_periodic_flow(initial_velocity=init_vel, relaxation_rate=1.95)
     with pytest.raises(ValueError) as e:
         shear_flow_scenario.run_iterative_initialization(max_steps=20000, check_residuum_after=500)
-    assert 'did not converge' in str(e)
+    assert 'did not converge' in str(e.value)
 
     shear_flow_scenario = create_fully_periodic_flow(initial_velocity=init_vel, relaxation_rate=1.6)
     shear_flow_scenario.run_iterative_initialization(max_steps=20000, check_residuum_after=500)

lbmpy_tests/test_relaxation_rate.py

@@ -9,7 +9,7 @@ def test_relaxation_rate():
                               relaxation_rates=[1 + i / 10 for i in range(19)])
     with pytest.raises(ValueError) as e:
         get_shear_relaxation_rate(method)
-    assert 'Shear moments are relaxed with different relaxation' in str(e)
+    assert 'Shear moments are relaxed with different relaxation' in str(e.value)
 
     method = create_lb_method(stencil="D2Q9", method='mrt_raw',
                               relaxation_rates=[1 + i / 10 for i in range(9)])

lbmpy_tests/test_stencils.py

@@ -67,7 +67,7 @@ def test_free_functions():
 
     with pytest.raises(ValueError) as e:
         get_stencil("name_that_does_not_exist")
-    assert "No such stencil" in str(e)
+    assert "No such stencil" in str(e.value)
 
 
 def test_visualize():