Move tox basepython to top-level

Currently we're building docs with python 2.  It works fine with
python3, so move the basepython override up to the top level so we're
all-python3-all-the-time.

Change-Id: I2c49a0b8bcda72385a9594f40a6b9db10403d688

tox.ini

@@ -4,12 +4,12 @@ envlist = linters
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:linters]
-basepython = python3
 whitelist_externals = bash
 commands =
   flake8