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

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

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

Change-Id: I660a50d89af2cccdd23c161daf581ebdb89ce9e8
Sem-Ver: api-break

releasenotes/notes/drop-python27-support-87786dbabd832d89.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Support for Python 2.7 has been dropped. The minimum version of Python now
+    supported is Python 3.6.

{{cookiecutter.repo_name}}/setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://www.openstack.org/
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,6 +16,8 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3 :: Only
+    Programming Language :: Python :: Implementation :: CPython
 
 [files]
 packages =

{{cookiecutter.repo_name}}/tox.ini

@@ -1,9 +1,12 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = py37,pep8
 skipsdist = True
+ignore_basepython_conflict = true
+
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv =
    PYTHONWARNINGS=default::DeprecationWarning
@@ -15,7 +18,6 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
 commands = stestr run {posargs}
 
 [testenv:lower-constraints]
-basepython = python3
 deps = -c{toxinidir}/lower-constraints.txt
        -r{toxinidir}/test-requirements.txt