Fix import: sympy.numbers -> sympy.core.numbers

Apparently sympy no longer exports sympy.numbers directly.

pystencils/sympyextensions.py

@@ -261,7 +261,7 @@ def subs_additive(expr: sp.Expr, replacement: sp.Expr, subexpression: sp.Expr,
             return current_expr
         else:
             if current_expr.func == sp.Mul and sp.core.numbers.Zero() in param_list:
-                return sp.n.core.numbers.Zero()
+                return sp.core.numbers.Zero()
             else:
                 return current_expr.func(*param_list, evaluate=False)