Replace deprecated function

assertRaisesRegexp is deprecated because in unittest version 3.2 was renamed
to assertRaisesRegex.

Change-Id: I0454200dc730fd4d3f0e1b234d48d8875c843fb6
Closes-Bug: #1683818
diff --git a/tempest/api/compute/flavors/test_flavors_negative.py b/tempest/api/compute/flavors/test_flavors_negative.py
index b2c33e2..91e9684 100644
--- a/tempest/api/compute/flavors/test_flavors_negative.py
+++ b/tempest/api/compute/flavors/test_flavors_negative.py
@@ -82,9 +82,9 @@
         self.assertEqual(min_img_ram, image['min_ram'])
 
         # Try to create server with flavor of insufficient ram size
-        self.assertRaisesRegexp(lib_exc.BadRequest,
-                                "Flavor's memory is too small for "
-                                "requested image",
-                                self.create_test_server,
-                                image_id=image['id'],
-                                flavor=flavor['id'])
+        self.assertRaisesRegex(lib_exc.BadRequest,
+                               "Flavor's memory is too small for "
+                               "requested image",
+                               self.create_test_server,
+                               image_id=image['id'],
+                               flavor=flavor['id'])