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: I798f7f0a99da5ee8ae848049dcd08a36b495bb62

tox.ini

@@ -14,7 +14,6 @@ minversion = 2.3
 skipsdist = True
 
 [testenv]
-basepython = python3
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
@@ -35,6 +34,7 @@ deps = -r{toxinidir}/requirements.txt
        {[testenv]tsconfig_dir}
 
 [testenv:linters]
+basepython = python3
 whitelist_externals = bash
 commands =
   bash -c "find {toxinidir}        \
@@ -61,6 +61,7 @@ exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build,release-tag-*
 
 # Use flake8 to replace pep8.
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 skip_install = True
 deps =
@@ -69,6 +70,7 @@ commands =
     flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [nosetests]
@@ -130,6 +132,7 @@ whitelist_externals =
     reno
 
 [testenv:newnote]
+basepython = python3
 # Re-use the releasenotes venv
 envdir = {toxworkdir}/releasenotes
 deps = -r{toxinidir}/doc/requirements.txt