tox: Trivial cleanup

Remove some noise and move 'basepython' to the top-level 'testenv'. Also
use the new-style URL for upper constraints.

Change-Id: I1a5f9bbcad2ef9e4551739900a7e688bfffa0a6c
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -1,56 +1,47 @@
 [tox]
-minversion = 2.0
-distribute = False
+minversion = 3.1
 envlist = py27,py37,pep8
+ignore_basepython_conflict = True
 
 [testenv]
-install_command = pip install {opts} {packages}
+basepython = python3
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
-basepython = python3
 commands = flake8
 
 [testenv:cover]
-basepython = python3
 setenv =
-    PYTHON=coverage run --source oslotest --parallel-mode
+  PYTHON=coverage run --source oslotest --parallel-mode
 commands =
-    stestr run {posargs}
-    coverage combine
-    coverage html -d cover
-    coverage xml -o cover/coverage.xml
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
-[flake8]
-show-source = True
-exclude = .tox,dist,doc,*.egg,build
-
-[hacking]
-import_exceptions =
-    six.moves.mock
-
 [testenv:releasenotes]
-basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[flake8]
+show-source = true
+exclude = .tox,dist,doc,*.egg,build
+
+[hacking]
+import_exceptions =
+  six.moves.mock