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: I8f1883011c0831b78a5f099e1ddcf98a77e4005a

tox.ini

@@ -4,7 +4,6 @@ envlist = checkbuild
 skipsdist = True
 
 [testenv]
-basepython = python2
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
@@ -15,6 +14,7 @@ whitelist_externals =
   rsync
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [doc8]