tox: Remove ineffective ignore_basepython_conflict and bump minimum version

Change-Id: Ia5940434b593dc993bbd9fef11b483475f8a96db
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,11 +1,9 @@
 [tox]
-minversion = 4.0
+minversion = 4.6.0
 skipsdist = True
 envlist = docs,molecule
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = False
 commands =
     /usr/bin/find . -type f -name "*.pyc" -delete
@@ -61,7 +59,6 @@ commands =
 commands =
     {posargs}
 
-
 [testenv:molecule]
 # You can use DOCKER_REGISTRY and DOCKER_IMAGE_TAG to switch between
 # tested distros. I.e: