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 .

Change-Id: I9ae1123fbdd823bc43eefbd78e80b9a1adea637e
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -1,10 +1,8 @@
 [tox]
 minversion = 4.2.5
 envlist = py3,pep8
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}