tox: Remove basepython and ineffective ignore_basepython_conflict

Change-Id: Id041630a143ee07e9bdc1acbb0ab0cea66dc0543
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

Author: anfimovir

tox.ini

@@ -2,10 +2,8 @@
 minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} -U {opts} {packages}
 setenv =