Skip test_model_graph if pydot is not installed

As pydot doesn't always install correctly, we don't want the test to fail
because of it.

proteus/tests/test_report.py

@@ -1,11 +1,18 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
+import unittest
+try:
+    import pydot
+except ImportError:
+    pydot = None
+
 from proteus import Report, Model
 from .common import ProteusTestCase
 
 
 class TestReport(ProteusTestCase):
 
+    @unittest.skipIf(not pydot, 'requires pydot')
     def test_model_graph(self):
         IrModel = Model.get('ir.model')
         models = IrModel.find([])