Bugfix for MultiWidget.removeWidgets on Python 3

You can't iterate over zip() twice in Python 3.  This used to clear
self.widgets completely on Python 3.  Luckily tests caught it.

Fixes #13.

src/z3c/form/widget.py

@@ -346,7 +346,7 @@ def removeWidgets(self, names):
         :param names: list of widget.name to remove from the value
         :return: None
         """
-        zipped = zip(self.key_widgets,self.widgets)
+        zipped = list(zip(self.key_widgets,self.widgets))
         self.key_widgets = [k for k,v in zipped if v.name not in names]
         self.widgets = [v for k,v in zipped if v.name not in names]
         if self.is_dict: