Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I4869426a4c39071a9e6d60c02ed13f8e5bb72536

coverage2sql/tests/test_shell.py

@@ -13,8 +13,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest import mock
+
 import fixtures
-import mock
 from oslo_config import cfg
 import tempfile