Declare real and imag as read-only attributes.

This fixes SF bug #514858 (Gregory Smith): complex not entirely
immutable

2.2.1 Bugfix candidate!

Objects/complexobject.c

@@ -629,9 +629,9 @@ static PyMethodDef complex_methods[] = {
 };
 
 static PyMemberDef complex_members[] = {
-	{"real", T_DOUBLE, offsetof(PyComplexObject, cval.real), 0,
+	{"real", T_DOUBLE, offsetof(PyComplexObject, cval.real), READONLY,
 	 "the real part of a complex number"},
-	{"imag", T_DOUBLE, offsetof(PyComplexObject, cval.imag), 0,
+	{"imag", T_DOUBLE, offsetof(PyComplexObject, cval.imag), READONLY,
 	 "the imaginary part of a complex number"},
 	{0},
 };