Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Change-Id: I781b979d11c3b974701ae38574899e6160fdc6bb

lower-constraints.txt

@@ -28,7 +28,6 @@ keystonemiddleware==4.17.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monasca-common==2.7.0
 monasca-statsd==1.1.0
 monotonic==0.6

monasca_log_api/tests/test_request.py

@@ -12,8 +12,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from unittest import mock
+
 from falcon import testing
-from mock import mock
 
 from monasca_log_api.app.base import request
 from monasca_log_api.app.base import validation