ServiceNow expression and configuration syntax can contain nested parentheses, square brackets, and braces. Given a string containing only the bracket characters (), [], and {}, determine whether the brackets are correctly matched and properly nested.
A string is valid when every opening bracket has the corresponding closing bracket, closing brackets appear in last-in, first-out order, and no closing bracket appears without a matching opener.
Implement is_valid_brackets(s), where s is a string. Return True if all brackets are valid and False otherwise. The input contains only (, ), [, ], {, and }.
def is_valid_brackets(s):