[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/pyvoltha/adapters/extensions/events/onu/onu_equipment_alarm.py b/pyvoltha/adapters/extensions/events/onu/onu_equipment_alarm.py
index 0c5fee4..e624ba0 100644
--- a/pyvoltha/adapters/extensions/events/onu/onu_equipment_alarm.py
+++ b/pyvoltha/adapters/extensions/events/onu/onu_equipment_alarm.py
@@ -11,6 +11,7 @@
 # WITHOUT 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 __future__ import absolute_import
 from voltha_protos.events_pb2 import AlarmEventType, AlarmEventSeverity, AlarmEventCategory
 from pyvoltha.adapters.extensions.alarms.adapter_alarms import AlarmBase