tox: Bump minimum version and 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: Id656bf70a807f022bdf1fde368655a4b57667b6d

tox.ini

@@ -1,12 +1,10 @@
 [tox]
-minversion = 3.1
+minversion = 4.4
 skipsdist = True
 envlist = linters,docs,releasenotes,inventory,py3-inventory
-ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
-basepython = python3
 install_command =
     pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 deps =