Fix setting sign on RPython’s big integer representation

Signed-off-by: Stefan Marr <[email protected]>

src/som/compiler/parser.py

@@ -299,7 +299,7 @@ def _literal_integer(self, negate_value):
 
             bigint = bigint_from_str(self._text)
             if negate_value:
-                bigint.sign = -1
+                bigint._set_sign(-1)
             result = BigInteger(bigint)
         except ValueError:
             raise ParseError(