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

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

freezer-dr 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://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I8c2e92f459c6a0fda26b44039837bd95db55873e

.zuul.yaml

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

releasenotes/notes/drop-py-2-7-7c6101f30ffd8c71.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of freezer-dr
+    to support py2.7 is OpenStack Train. The minimum version of Python now
+    supported by freezer-dr is Python 3.6.

setup.cfg

@@ -14,8 +14,6 @@ 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 +1,10 @@
 [tox]
 minversion = 2.0
-envlist = py27,py37,pep8,pylint,docs
+envlist = py37,pep8,pylint,docs
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -33,17 +34,12 @@ python_files = test_*.py
 norecursedirs  = .tox .venv
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands =
   python setup.py build_sphinx
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:py36]
 basepython = python3.6
 
@@ -51,15 +47,12 @@ basepython = python3.6
 basepython = python3.7
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 freezer_dr
 
 [testenv:pylint]
-basepython = python3
 commands = pylint --rcfile .pylintrc freezer_dr
 
 [testenv:genconfig]
-basepython = python3
 sitepackages = False
 envdir = {toxworkdir}/venv
 commands =
@@ -78,7 +71,6 @@ show-source = True
 exclude = .venv,.tox,dist,doc,test,*egg,tests
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt