Pin pylint version to 3.1.1 in tox.ini

Change-Id: Id2503e4e58c5f75c9aebdb3722630bfce7e42051
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/tox.ini b/tox.ini
index 8b329bf..0345319 100644
--- a/tox.ini
+++ b/tox.ini
@@ -64,7 +64,7 @@
 deps =  {[testenv]deps}
         -r{toxinidir}/requirements-dev.txt
         -r{toxinidir}/requirements-test.txt
-        pylint
+        pylint==3.1.1
 commands =
         pylint -E osmclient