[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: I2bcf1208d75d8cdda4e3892b3e48c85eaf5db8d1

.zuul.yaml

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

setup.py

@@ -64,10 +64,10 @@ 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/charms.ceph',

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py3
+envlist = pep8,py3
 skipsdist = True
 skip_missing_interpreters = True
 
@@ -11,13 +11,6 @@ commands =
   stestr run {posargs}
 sitepackages = False
 
-[testenv:py27]
-# ceph charms are Python3-only, but py27 unit test target
-# is required by OpenStack Governance.  Remove this shim as soon as
-# permitted.  http://governance.openstack.org/reference/cti/python_cti.html
-whitelist_externals = true
-commands = true
-
 [testenv:py3]
 basepython = python3
 deps = -r{toxinidir}/requirements.txt