[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

Watcher is ready with python 3 and ok to drop the
python 2.7 support.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Depends-On: https://review.opendev.org/#/c/693631/

Change-Id: I603c6d2c22779e8ef2e70eb6369fc521a77c9c3a

requirements.txt

@@ -3,7 +3,6 @@
 # process, which may cause wedges in the gate later.
 
 apscheduler>=3.5.1 # MIT License
-enum34>=1.1.6;python_version=='2.7' or python_version=='2.6' or python_version=='3.3' # BSD
 jsonpatch>=1.21 # BSD
 keystoneauth1>=3.4.0 # Apache-2.0
 jsonschema>=2.6.0 # MIT
@@ -46,8 +45,6 @@ stevedore>=1.28.0 # Apache-2.0
 taskflow>=3.1.0 # Apache-2.0
 WebOb>=1.8.5 # MIT
 WSME>=0.9.2 # MIT
-# NOTE(fdegir): NetworkX 2.3 dropped support for Python 2
-networkx>=1.11,<2.3;python_version<'3.0'  # BSD
 networkx>=1.11;python_version>='3.4' # BSD
 microversion_parse>=0.2.1 # Apache-2.0
 futurist>=1.8.0 # Apache-2.0