Drop most of six importings

This update is to drop six, and also Python2 support. However, only
`tacker/test/common/helpers.py` which defines a set of functions for
Python2 is remained because `compact_byte()` is called from several test
classes. Although it'is not so quite hard to drop, better to be done in
another patch because so many changes shouldn't be included in a update.

Here is a summary of changes.

* Remove all six importing other than `helpers.py`.

* Revise the order of importing modules becaseu it's checked strictly
  more in the recent pep8 test.

* For the update, it removes a test `test_json_with_utf()` defined in
  `tacker/tests/unit/test_wsgi.py` because it's required only for Python2
  and can be skipped for Python3, so no need to keep anymore.

* Remove six from `requirements.txt` and `lower-constraints.txt`.

Partial-bug: #1778644

Change-Id: Ib37fd00ab6836e4efc04d1c14c10f312379c7212
Signed-off-by: Yasufumi Ogawa <yasufum.o@gmail.com>

tacker/alarm_receiver.py

@@ -11,10 +11,13 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+
+from urllib import parse
+
 from oslo_config import cfg
 from oslo_log import log as logging
 from oslo_serialization import jsonutils
-from six.moves.urllib import parse
+
 from tacker._i18n import _
 from tacker.vnfm.monitor_drivers.token import Token
 from tacker import wsgi