Replace deprecated assertEquals

The assertEquals method has been deprecated since it was renamed
to assertEqual in Python 3.2.

https://docs.python.org/3/library/unittest.html#deprecated-aliases

Change-Id: I559ad57c2d96a3bb51080c72816f39d3baa4c9dd

diskimage_builder/elements/growvols/tests/test_growvols.py

@@ -283,7 +283,7 @@ class TestGrowvols(base.BaseTestCase):
         # No partitions
         e = self.assertRaises(Exception, growvols.find_next_device_name,
                               devices, 'sdb', 1)
-        self.assertEquals(
+        self.assertEqual(
             'Could not find partition naming scheme for sdb', str(e))
 
         # Use NVMe device naming