disable optimised sequence unpacking for bytes/str - hard to get right for Py2/Py3

Cython/Compiler/ParseTreeTransforms.py

@@ -432,9 +432,7 @@ def flatten_parallel_assignments(input, output):
     #  individual elements.  This transformation is applied
     #  recursively, so that nested structures get matched as well.
     rhs = input[-1]
-    if (not (rhs.is_sequence_constructor or
-             (rhs.is_string_literal and not (rhs.type.is_string or
-                                             rhs.type is Builtin.bytes_type)))
+    if (not (rhs.is_sequence_constructor or isinstance(rhs, ExprNodes.UnicodeNode))
         or not sum([lhs.is_sequence_constructor for lhs in input[:-1]])):
         output.append(input)
         return