Message5268

I've confirmed that in the original code a list was passed to the
'value' field.  In python this didn't cause any problems, despite the
fact that the official AST description for the value field of an Assign
is for a single expression, and not for a list of expressions.  The type
checking in Java caused the bad assignment to default to None.

Fixing this issue would only be useful if Jython needs to handle badly
formed AST trees the same way as Python.  Otherwise, raising an
exception for a bad assignment might be worth while.