Try to make code in test_check_equal compatible with matplotlib 2.0.2

Author: weiji14

tests/test_pytest_mpl.py

@@ -232,9 +232,11 @@ def test_succeeds(self):
 @pytest.mark.mpl_image_compare
 def test_check_equal():
     fig_test = plt.figure()
-    fig_test.subplots().plot([1, 3, 5])
+    ax_test = fig_test.add_subplot()
+    ax_test.plot([1, 3, 5])
 
     fig_ref = plt.figure()
-    fig_ref.subplots().plot([0, 1, 2], [1, 3, 5])
+    ax_ref = fig_ref.add_subplot()
+    ax_ref.plot([0, 1, 2], [1, 3, 5])
 
     return fig_test, fig_ref