[ussuri][goal] Updates for python 2.7 drop

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

charm repos need few updates on either py2 drop or updating
ussuri python template, tox.ini etc.

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

Needed-By: https://review.opendev.org/#/c/714728/

Change-Id: I6cb655295287c367e128194f1264910a2d761e79

.zuul.yaml

@@ -1,4 +1,3 @@
 - project:
     templates:
-      - python-charm-jobs
       - openstack-python3-ussuri-jobs

setup.py

@@ -57,10 +57,10 @@ setup(
         "Topic :: System",
         "Topic :: System :: Installation/Setup",
         "Topic :: System :: Software Distribution",
-        "Programming Language :: Python :: 2",
-        "Programming Language :: Python :: 2.7",
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3.5",
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
         "License :: OSI Approved :: Apache Software License",
     ],
     url='https://github.com/openstack-charmers/charms.openstack',

tox.ini

@@ -10,13 +10,6 @@ install_command =
   pip install {opts} {packages}
 commands = stestr run {posargs}
 
-[testenv:py27]
-basepython = python2.7
-deps = -r{toxinidir}/test-requirements.txt
-# Py27 needs to be disabled as upstream charms.reactive is not Py3.5+ only
-# However, we can't yet remove the actually py27 test from the gate.
-commands = python -c "print('Py27 testing disabled.')" && /bin/true
-
 [testenv:py3]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt