Drop python 2.7 support and testing

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

ceilometermiddleware 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://review.opendev.org/#/c/691178/

Change-Id: I48f75b701ecc4eee3134473bc854a0825b705203

.zuul.yaml

@@ -1,6 +1,5 @@
 - project:
     templates:
-      - openstack-python-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

releasenotes/notes/drop-py-2-7-21178ddd27b28806.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of ceilometermiddleware
+    to support python 2.7 is OpenStack Train. The minimum version of Python now
+    supported by ceilometermiddleware is Python 3.6.

setup.cfg

@@ -13,8 +13,6 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.8
-envlist = py27,py37,pypy,pep8,releasenotes
+envlist = py37,pypy,pep8,releasenotes
 skipsdist = True
 
 [testenv]