Add explicitly sale module as tests_require

Since issue10375 stock_package_shipping does not depend anymore on
sale_shipment_cost. So sale is no more an implicit dependency.

setup.py

@@ -63,7 +63,10 @@ for dep in info.get('depends', []):
         requires.append(get_require_version('trytond_%s' % dep))
 requires.append(get_require_version('trytond'))
 
-tests_require = [get_require_version('proteus')]
+tests_require = [
+    get_require_version('proteus'),
+    get_require_version('trytond_sale'),
+    ]
 dependency_links = []
 if minor_version % 2:
     dependency_links.append(