tox: Drop basepython

Python 2 reached its EOL long time ago and we no longer expect any
user may attempt to run tox in Python 2.

Removing the option allows us to remove ignore_basepython_conflict and bump minversion.

Change-Id: Ib28c39de188d42777d1769bb0973cd3324118b28
Co-authored-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,8 +1,7 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py36,pypy,pep8
 skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
@@ -13,7 +12,6 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
-basepython = python3
 
 [testenv:pep8]
 commands = flake8 {posargs}