Drop python 2.7 support and testing

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

python-aodhclient 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: I734a2b525df0b3fc8ddf16a62025fab5f66a01aa

.zuul.yaml

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

releasenotes/notes/drop-py-2-7-8f26d7e1e8dc83c2.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of python-aodhclient
+    to support py2.7 is OpenStack Train. The minimum version of Python now
+    supported by python-aodhclient 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

@@ -1,9 +1,11 @@
 [tox]
-minversion = 1.6
-envlist = py27,py37,pep8
+minversion = 3.1.1
+envlist = py37,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
@@ -19,18 +21,15 @@ deps =
 commands = pifpaf run aodh -- stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 deps = hacking<0.13,>=0.12
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 # Used by openstack-infra to build the doc
 deps = -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source aodhclient --parallel-mode
@@ -42,11 +41,9 @@ commands =
    coverage report
 
 [testenv:releasenotes]
-basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
    rm -rf doc/build
@@ -54,7 +51,6 @@ commands =
 whitelist_externals = rm
 
 [testenv:debug]
-basepython = python3
 commands = pifpaf --debug run aodh -- oslo_debug_helper {posargs}
 
 [flake8]