tox: Trivial cleanup

1. refactor basepython to testenv
2. remove py27 target
3. fix a bug that prevented usage of upper constraints in some test envs

Change-Id: I5fcec1c7ec978f9fb279de8ac9aae227e6ec5e9d

tox.ini

@@ -1,9 +1,10 @@
 [tox]
-minversion = 2.0
-envlist = py27,py37,pep8
+minversion = 3.1
+envlist = py37,pep8
+ignore_basepython_conflict = True
 
 [testenv]
-install_command = pip install {opts} {packages}
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -15,12 +16,9 @@ commands =
   stestr run --slowest (?!tests.test_functional_eventlet)tests {posargs}
   env TEST_EVENTLET=1 stestr run --slowest tests.test_functional_eventlet
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
-basepython = python3
 deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
 commands =
   flake8
@@ -28,7 +26,6 @@ commands =
   bandit -r oslo_rootwrap tests -n5 --skip B404
 
 [testenv:cover]
-basepython = python3
 deps = {[testenv]deps}
     coverage
 setenv =
@@ -40,13 +37,13 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 whitelist_externals = rm
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
   rm -fr doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html
@@ -63,19 +60,18 @@ basepython = python2.7
 commands = python benchmark/benchmark.py
 
 [testenv:benchmark3]
-basepython = python3
 commands = python3 benchmark/benchmark.py
 
 [testenv:releasenotes]
-basepython = python3
 whitelist_externals = rm
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt