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: I4a4959ab6486405ddc6b3b5bee8142b2afce259f

tox.ini

@@ -4,7 +4,6 @@ minversion = 2.3
 skipsdist = True
 
 [testenv]
-basepython = python3
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
@@ -13,6 +12,7 @@ setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:linters]
+basepython = python3
 whitelist_externals = bash
 commands =
   bash -c "find {toxinidir}          \
@@ -31,6 +31,7 @@ commands =
 ####
 # Add flake8 as pep8 codestyle check.
 [testenv:pep8]
+basepython = python3
 description =
   Run style checks.
 commands =
@@ -45,6 +46,7 @@ ignore = E123,E125,E501,H405
 exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build,release-tag-*
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
@@ -66,6 +68,7 @@ whitelist_externals =
     reno
 
 [testenv:newnote]
+basepython = python3
 # Re-use the releasenotes venv
 envdir = {toxworkdir}/releasenotes
 deps = -r{toxinidir}/doc/requirements.txt