Drop python 2.7 support and testing

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

Co-Authored-By: Andreas Jaeger <aj@suse.com>
Change-Id: I32d5dc317b821dee4a625506f227a8f5bfaaecfc

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - check-requirements
       - release-notes-jobs-python3
-      - openstack-python-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - openstack-lower-constraints-jobs

releasenotes/notes/drop-py2-7-c7fce3322cc66380.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of openstack-doc-tools
+    to support python 2.7 is 2.0.0. The minimum version of Python now
+    supported by openstack-doc-tools is Python 3.6.

requirements.txt

@@ -6,7 +6,6 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 iso8601>=0.1.11 # MIT
 lxml!=3.7.0,>=3.4.1 # BSD
 docutils>=0.11 # OSI-Approved Open Source, Public Domain
-sphinx>=1.8.0,<2.0.0;python_version=='2.7' # BSD
-sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
+sphinx>=1.8.0,!=2.1.0 # BSD
 demjson>=2.2.2 # GLGPLv3+
 PyYAML>=3.12 # MIT

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 = 3.1.0
+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}
 # Install also sitemap scraping tool, not installed by default
@@ -15,7 +20,6 @@ deps = scrapy>=1.0.0
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands =
     flake8
     # Run doc8 to check .rst and .txt files.
@@ -27,7 +31,6 @@ commands =
     bashate bin/doc-tools-check-languages bin/doc-tools-build-rst
 
 [testenv:releasenotes]
-basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
@@ -35,11 +38,9 @@ commands =
 # commands = functional test command goes here
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands =
   sphinx-build -W --keep-going -b html -d doc/build/doctrees doc/source doc/build/html
 
@@ -50,7 +51,6 @@ commands =
 # separately, outside of the requirements files, and develop mode disabled
 # explicitly to avoid unnecessarily installing the checked-out repo too (this
 # further relies on "tox.skipsdist = True" above).
-basepython = python3
 deps = bindep
 commands = bindep test
 usedevelop = False
@@ -64,7 +64,6 @@ max-complexity=29
 ignore = H101
 
 [testenv:lower-constraints]
-basepython = python3
 # Install also sitemap scraping tool, not installed by default
 # therefore not in requirements file
 deps =