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: I7428f118c92f6a4c67bdb2c855df1a933587518b

tox.ini

@@ -4,7 +4,6 @@ envlist = checkbuild
 skipsdist = True
 
 [testenv]
-basepython = python2.7
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} -U {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
@@ -17,6 +16,7 @@ basepython = python3
 commands = {posargs}
 
 [testenv:checkbuild]
+basepython = python3
 setenv =
   IGNORE_GIT=1
 commands =