Update test-requirements to be py3 only

This commit updates the test-requirements to remove things no longer
needed in a py3 only world. This involves updating the sphinx version so
that we don't have to explicitly set a python 2 version. It also means
removing the mock library as we can just use mock from stdlib now that
we're only supporting python3.

stestr/tests/repository/test_util.py

@@ -10,10 +10,10 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
 import os
 import shutil
 import tempfile
+from unittest import mock
 
 from stestr.repository import util
 from stestr.tests import base

stestr/tests/test_bisect_tests.py

@@ -12,8 +12,8 @@
 
 import io
 import operator
+from unittest import mock
 
-import mock
 import subunit
 import testtools
 

stestr/tests/test_config_file.py

@@ -10,8 +10,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from unittest import mock
+
 import ddt
-import mock
 
 from stestr import config_file
 from stestr.tests import base

stestr/tests/test_scheduler.py

@@ -12,8 +12,8 @@
 
 import datetime
 import re
+from unittest import mock
 
-import mock
 from subunit import iso8601
 
 from stestr.repository import memory

stestr/tests/test_selection.py

@@ -12,8 +12,7 @@
 
 import io
 import re
-
-import mock
+from unittest import mock
 
 from stestr import selection
 from stestr.tests import base

stestr/tests/test_subunit_trace.py

@@ -17,11 +17,11 @@
 import io
 import os
 import sys
+from unittest.mock import patch
 
 from ddt import data
 from ddt import ddt
 from ddt import unpack
-from mock import patch
 
 from stestr import subunit_trace
 from stestr.tests import base

stestr/tests/test_test_processor.py

@@ -11,8 +11,7 @@
 # under the License.
 
 import subprocess
-
-import mock
+from unittest import mock
 
 from stestr import test_processor
 from stestr.tests import base

stestr/tests/test_user_config.py

@@ -12,8 +12,7 @@
 
 import io
 import os
-
-import mock
+from unittest import mock
 
 from stestr.tests import base
 from stestr import user_config

test-requirements.txt

@@ -3,9 +3,7 @@
 # process, which may cause wedges in the gate later.
 
 hacking<1.2.0,>=1.1.0
-sphinx!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
-sphinx!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4'  # BSD
-mock>=2.0 # BSD
+sphinx>2.1.0 # BSD
 subunit2sql>=1.8.0
 coverage>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT