Set min version of tox to 3.2.1

As recommended, since version 3.2.0 tox switches pip invocations
to use the module -m pip instead of direct invocation.
We set min version to 3.2.1 [1] to also fix the behavior of
--parallel--safe-build

[1] https://tox.readthedocs.io/en/latest/changelog.html#v3-2-1-2018-08-10

Change-Id: I4003a8bebc7737250d0c318426e885d0888df57d

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.0
+minversion = 3.2.1
 skipsdist = True
 envlist = py3,pep8
 ignore_basepython_conflict=true