Drop py2 support and testing

...per Ussuri Communtiy-wide goal:
    https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

At the same time, set ignore_basepython_conflict

Automatic envs (pyXX) will only use the python version appropriate to
that env and ignore basepython inherited from [testenv] if we set
ignore_basepython_conflict.

Change-Id: I5ab48fe70e49d104290f1c01014e4aa6420e300f

.zuul.yaml

@@ -2,5 +2,4 @@
     templates:
       - check-requirements
       - publish-openstack-docs-pti
-      - openstack-python-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs

releasenotes/notes/drop-py2-support-3c3bb1055b271608.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. The minimum version of Python now
+    supported by sphinx-feature-classification 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 +1,14 @@
 [tox]
-minversion = 2.0
-envlist = py27,py37,pep8
+minversion = 3.1.1
+envlist = py37,pep8
 skipsdist = True
+# Automatic envs (pyXX) will only use the python version appropriate to that
+# env and ignore basepython inherited from [testenv] if we set
+# ignore_basepython_conflict.
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -15,28 +20,22 @@ deps =
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
-basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]