Use converted tests converted by 2to3

distutils import tests module from the source code, so we must force it to use
the test suite from the absolute path because it will be the converted on in
case of 2to3 otherwise we fall-back to the relative module.

issue3211

tests/__init__.py

@@ -1,6 +1,9 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 
-from .test_purchase import suite
+try:
+    from trytond.modules.purchase.tests.test_purchase import suite
+except ImportError:
+    from .test_purchase import suite
 
 __all__ = ['suite']