use S(0) instead of Zero

Author: latot

inst/@sym/diff.m

@@ -70,7 +70,7 @@
           'args = _ins[1:]'
           'd = list(f.free_symbols)'
           'if len(d) == 0:'
-          '    return sympy.Zero,'
+          '    return sympy.S(0),'
           'if len(args) == 0:'
           '    if len(d) > 1:'
           '        return ("NO_DEV_VAR"),'