Drop redundant injection of VIRTUAL_ENV

tox automatically injects the environment so no manual override is
necessary.

Also remove basepython because 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: I40eaf913140c9ed0f3b182f9ea15477e928f5bf5

tox.ini

@@ -2,13 +2,10 @@
 minversion = 3.18.0
 envlist = py3,pypy,pep8
 skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 setenv =
-   VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
 deps =
    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}