Remove Python 2.7 support from testing and gates

Makes Python 3 the base python for all tox venvs and removes
py2 jobs from zuul.yaml.

Remaining:
- move the legacy gate jobs from openstack-zuul-jobs to the os-brick
  repo and make them py3 jobs
- adjust setup.cfg to require py3 and remove py2 requirements from
  test-requirements.txt and doc/requirements.txt

Partial-bug: #1853372

Change-Id: Ib447656aa213914bafd50380b3821090f93776f8

.zuul.yaml

@@ -1,10 +1,8 @@
 - project:
     templates:
       - check-requirements
-      - lib-forward-testing
       - lib-forward-testing-python3
       - openstack-lower-constraints-jobs
-      - openstack-python-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

releasenotes/notes/drop-py2-7dcde3ccd0e167b0.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Beginning with os-brick
+    release 3.0.0, the minimum version of Python supported by
+    os-brick 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 +1,15 @@
 [tox]
-minversion = 2.0
-envlist = py27,py37,pep8
+minversion = 3.1.0
 skipsdist = True
+skip_missing_interpreters = true
+# python runtimes: https://governance.openstack.org/tc/reference/runtimes/ussuri.html
+envlist = py37,py36,pep8
+# this allows tox to infer the base python from the environment name
+# and override any basepython configured in this file
+ignore_basepython_conflict=true
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv =
     VIRTUAL_ENV={envdir}
@@ -21,7 +27,7 @@ deps =
 # By default stestr will set concurrency
 # to ncpu, to specify something else use
 # the concurrency=<n> option.
-# call ie: 'tox -epy27 -- --concurrency=4'
+# call example: 'tox -epy37 -- --concurrency=4'
 commands =
     stestr run {posargs}
     stestr slowest
@@ -31,23 +37,19 @@ whitelist_externals = bash
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:debug]
-basepython = python3
 commands =
   find . -type f -name "*.pyc" -delete
   oslo_debug_helper {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:fast8]
-basepython = python3
 envdir = {toxworkdir}/pep8
 commands =
     {toxinidir}/tools/fast8.sh
 
 [testenv:pylint]
-basepython = python3
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
@@ -55,11 +57,9 @@ deps =
 commands = bash tools/lintstack.sh
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 # To see the report of missing coverage add to commands
 #   coverage report --show-missing
 setenv =
@@ -72,7 +72,6 @@ commands =
     coverage xml -o cover/coverage/xml
 
 [testenv:docs]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -83,7 +82,6 @@ whitelist_externals = rm
 
 
 [testenv:pdf-docs]
-basepython = python3
 deps = {[testenv:docs]deps}
 commands = {[testenv:docs]commands}
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -92,7 +90,6 @@ whitelist_externals =
   make
 
 [testenv:releasenotes]
-basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]
@@ -119,7 +116,6 @@ max-complexity=30
 import_exceptions = os_brick.i18n
 
 [testenv:bindep]
-basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
@@ -131,7 +127,6 @@ commands = bindep test
 usedevelop = False
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt