[VOL-2241] Python 3 refactor of pyvoltha

Majority of work was manual fixes to bytes and strings types, which are
different in py3, but same in py2. As the OMCI library does a lot of
these comparisons and scapy then renders packets, this was frequently
nontrival to debug.

Also:

- Removed grpc dep which wasn't being used, not py3 compatible
- s/Alarms/Events/ to work with protobuf changes per VOL-2224
- Automatic fixes via modernize tooling
- Removed unused OrderedWeakValueDict code
- Removed frameio send_frame specific to Darwin (MacOS), which had no
  corresponding linux code
- Use library functions for hex and unicode conversions
- Various other cleanups and fixes (EOL whitespace, etc.)

Also more (Matt):

 - handle stringify better, check if already string
 - use binary string for binary work
 - import new thread paths
 - update requirements.txt for newer libraries needed with newer python
 - return proper tuple for unpacking
 - bytes string formatting fixed
 - fix mock task unit test

Even more (Zack):

- Python 2/3 compat for _thread by using 'future'
- Bump version to 2.3.0

Change-Id: I53b596d374a944bfb80d0b112f21bcc1f8bcee6e
diff --git a/test/unit/extensions/omci/mock/mock_task.py b/test/unit/extensions/omci/mock/mock_task.py
index aad0c60..7bafc17 100644
--- a/test/unit/extensions/omci/mock/mock_task.py
+++ b/test/unit/extensions/omci/mock/mock_task.py
@@ -13,9 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+from __future__ import absolute_import
 from pyvoltha.adapters.extensions.omci.tasks.task import Task
 from pyvoltha.common.utils.asleep import asleep
-from twisted.internet.defer import inlineCallbacks, failure
+from twisted.internet.defer import inlineCallbacks, failure, AlreadyCalledError, CancelledError
 from twisted.internet import reactor
 
 
@@ -87,8 +88,10 @@
 
             if self._success:
                 self.deferred.callback(self._value)
+            else:
+                self.deferred.errback(failure.Failure(self._value))
 
-            self.deferred.errback(failure.Failure(self._value))
-
+        except AlreadyCalledError as all:
+            pass
         except Exception as e:
             self.deferred.errback(failure.Failure(e))