Cox Automotive's Dealer.com services exchange payloads that may contain nested grouping markers. Implement a method that determines whether a string of brackets is properly nested and closed.
A string is valid when every opening bracket has the correct closing bracket, brackets close in last-in, first-out order, and no closing bracket appears without a matching opener. The supported bracket pairs are (), [], and {}. An empty string is valid.
Implement is_valid_brackets(s), where s is a string containing only the six bracket characters. Return True if the brackets are properly nested, otherwise return False.
def is_valid_brackets(s):