Replace deprecated method ``testtools.try_import``

The method ``testtools.try_import`` was removed in testtools 2.8.1
in [1].

[1]https://github.com/testing-cabal/testtools/commit/1ac2d55ffb61f46b8f0496347dba926a8c24b2d4

Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: I717ee29e41971eda79b5d226652bfa0f8b455fc3
diff --git a/neutron_tempest_plugin/scenario/test_dns_integration.py b/neutron_tempest_plugin/scenario/test_dns_integration.py
index d6bafef..2784dc1 100644
--- a/neutron_tempest_plugin/scenario/test_dns_integration.py
+++ b/neutron_tempest_plugin/scenario/test_dns_integration.py
@@ -13,10 +13,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import importlib
 import ipaddress
 
-import testtools
-
 from oslo_log import log
 from tempest.common import utils
 from tempest.common import waiters
@@ -37,9 +36,21 @@
 
 # Note(jh): Need to do a bit of juggling here in order to avoid failures
 # when designate_tempest_plugin is not available
-dns_base = testtools.try_import('designate_tempest_plugin.tests.base')
-dns_waiters = testtools.try_import('designate_tempest_plugin.common.waiters')
-dns_data_utils = testtools.try_import('designate_tempest_plugin.data_utils')
+try:
+    dns_base = importlib.import_module('designate_tempest_plugin.tests.base')
+except ImportError:
+    dns_base = None
+try:
+    dns_waiters = importlib.import_module(
+        'designate_tempest_plugin.common.waiters')
+except ImportError:
+    dns_waiters = None
+try:
+    dns_data_utils = importlib.import_module(
+        'designate_tempest_plugin.data_utils')
+except ImportError:
+    dns_data_utils = None
+
 
 if dns_base:
     DNSMixin = dns_base.BaseDnsV2Test
diff --git a/requirements.txt b/requirements.txt
index eee78f8..7c8b715 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -11,5 +11,5 @@
 tempest>=29.2.0 # Apache-2.0
 tenacity>=3.2.1 # Apache-2.0
 ddt>=1.0.1 # MIT
-testtools>=2.2.0 # MIT
+testtools>=2.8.4 # MIT
 debtcollector>=1.2.0 # Apache-2.0