Drop basepython

This was required to support env with both python 2 and 3 are available
but is no longer needed now since python 2 was retired long time ago.

Also drop the VIRTUAL_ENV environment which is now automatically set
by tox.

Change-Id: I1468baa8dce5383781a1affeafb09495053557e9
Co-authored-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -4,12 +4,11 @@ envlist = docs,pep8
 skipsdist = True
 
 [testenv]
-basepython = python3
 usedevelop = True
-setenv =
-   VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
 commands =