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: I1d1d2ab862725bed2585406e5f215450a42f225e

etcd3gw/tests/test_client.py

@@ -10,7 +10,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 from etcd3gw.client import Etcd3Client
 from etcd3gw.exceptions import Etcd3Exception

test-requirements.txt

@@ -13,5 +13,4 @@ testtools>=1.4.0 # MIT
 pifpaf>=0.10.0 # Apache-2.0
 nose>=1.3.7  # GNU LGPL
 pytest>=3.0.0  # MIT
-mock>=2.0.0 # BSD
 urllib3>=1.15.1  # MIT