remove explicit mock

The explicit mock package is not required if you can use unittest.mock
from python >= 3.3

Change-Id: I1e3a764b38be66b994d790768bc5eb9be4237444

pbr/tests/test_packaging.py

@@ -48,7 +48,10 @@ import tempfile
 import textwrap
 
 import fixtures
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import pkg_resources
 import six
 import testscenarios

pbr/tests/test_pbr_json.py

@@ -10,7 +10,10 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 
 from pbr import pbr_json
 from pbr.tests import base

test-requirements.txt

@@ -6,7 +6,6 @@ wheel>=0.32.0 # MIT
 fixtures>=3.0.0 # Apache-2.0/BSD
 hacking>=1.1.0,<4.0.0;python_version>='3.6' # Apache-2.0
 mock>=2.0.0,<4.0.0;python_version=='2.7' # BSD
-mock>=2.0.0;python_version>='3.6' # BSD
 six>=1.12.0 # MIT
 stestr>=2.1.0,<3.0;python_version=='2.7' # Apache-2.0
 stestr>=2.1.0;python_version>='3.0' # Apache-2.0