tox: Remove basepython

tox uses the python version where tox is installed by default, and we
no longer expect that the tests may run in env with python 2.

Change-Id: Ib55d7d713570a32e9e541ec23782610b9a1d6edb
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -2,10 +2,8 @@
 envlist = py3,pep8
 minversion = 3.18.0
 skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 userdevelop = True
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt