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: I7a791c8239ae54b74e714145a69e9af00c96bc6f
diff --git a/tox.ini b/tox.ini
index 28ac9e7..b5940a7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.0
+minversion = 3.2.1
 envlist = pep8
 skipsdist = True
 ignore_basepython_conflict=true