fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: Ibb8ce8e655643161b0b7d4e84f0c2897e9c6417c

tox.ini

@@ -25,10 +25,12 @@ commands =
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8 {posargs}
 
 [testenv:cover]
+basepython = python3
 # Also do not run test_coverage_ext tests while gathering coverage as those
 # tests conflict with coverage.
 commands =
@@ -39,14 +41,17 @@ commands =
   coverage html --include='compute_hyperv/*' --omit='compute_hyperv/openstack/common/*' -d covhtml -i
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands =
   python setup.py build_sphinx
   bash -c '! find doc/ -type f -name *.json | xargs -t -n1 python -m json.tool 2>&1 > /dev/null | grep -B1 -v ^python'
 
 [testenv:releasenotes]
+basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
@@ -70,6 +75,7 @@ enable-extensions = H904
 local-check-factory = compute_hyperv.hacking.checks.factory
 
 [testenv:pip-missing-reqs]
+basepython = python3
 # do not install test-requirements as that will pollute the virtualenv for
 # determining missing packages
 # this also means that pip-missing-reqs must be installed separately, outside