py: Rename MP_OBJ_NOT_SUPPORTED to MP_OBJ_NULL.

See issue #608 for justification.
diff --git a/py/objint_mpz.c b/py/objint_mpz.c
index 32c8595..2df3232 100644
--- a/py/objint_mpz.c
+++ b/py/objint_mpz.c
@@ -90,7 +90,7 @@
         case MP_UNARY_OP_POSITIVE: return o_in;
         case MP_UNARY_OP_NEGATIVE: { mp_obj_int_t *o2 = mp_obj_int_new_mpz(); mpz_neg_inpl(&o2->mpz, &o->mpz); return o2; }
         case MP_UNARY_OP_INVERT: { mp_obj_int_t *o2 = mp_obj_int_new_mpz(); mpz_not_inpl(&o2->mpz, &o->mpz); return o2; }
-        default: return MP_OBJ_NOT_SUPPORTED;
+        default: return MP_OBJ_NULL; // op not supported
     }
 }
 
@@ -108,7 +108,7 @@
         zlhs = &((mp_obj_int_t*)lhs_in)->mpz;
     } else {
         // unsupported type
-        return MP_OBJ_NOT_SUPPORTED;
+        return MP_OBJ_NULL;
     }
 
     // if rhs is small int, then lhs was not (otherwise mp_binary_op handles it)
@@ -213,7 +213,7 @@
                 break;
 
             default:
-                return MP_OBJ_NOT_SUPPORTED;
+                return MP_OBJ_NULL; // op not supported
         }
 
         return res;
@@ -233,7 +233,7 @@
                 return MP_BOOL(cmp == 0);
 
             default:
-                return MP_OBJ_NOT_SUPPORTED;
+                return MP_OBJ_NULL; // op not supported
         }
     }
 }