Set the value of a general constraint attribute. Note that, due to our lazy update approach, the change won't actually take effect until you update the model (using Model.update), optimize the model (using Model.optimize), or write the model to disk (using Model.write).
The full list of available attributes can be found in the Attributes section.
AttributeError if the specified attribute doesn't
exist or can't be set.
attrname: The attribute being modified.
newvalue: The desired new value of the attribute.