fixup! Add referred parts' hash strings in __buildout_signature__, that...

fixup! Add referred parts' hash strings in __buildout_signature__, that invokes rebuild of a part when one of its (recursive) dependencies are modified.

src/zc/buildout/tests.py

@@ -2100,7 +2100,7 @@ def test_part_pulled_by_recipe():
     ...         self.x = buildout[options['x']][name]
     ...
     ...     def install(self):
-    ...         print self.x
+    ...         print(self.x)
     ...         return ()
     ...
     ...     update = install