fix tuple unpacking bug

tests/run/tuplereassign.pyx

+__doc__ = """
+    >>> test1( (1,2,3) )
+    1
+    >>> test3( (1,2,3) )
+    3
+    >>> test( (1,2,3) )
+    3
+"""
+
+def test1(t):
+    t,a,b = t
+    return t
+
+def test3(t):
+    a,b,t = t
+    return t
+
+def test(t):
+    t,t,t = t
+    return t