No officer or employee of the city may have a financial interest, whether it be direct or indirect, including ownership of stock in any corporation, in any contract with the city or in the sale to the city, or to a contractor supplying the city, of any lands or rights or interest in any land, material, supplies or services. Any willful violation of this section shall constitute an offense against this charter and any officer or employee of the city, upon conviction thereof, shall thereby forfeit his office of employment. Any violation of this section with the knowledge, express or implied, of the person or corporation contracting with the city shall render the contract voidable by the city; provided, the provisions of this section shall not apply where the services, supplies, materials, lands or any other commodities are purchased or acquired pursuant to open and competitive bidding, conditioned upon the contract being awarded to the lowest and best bidder. This section shall not apply to ownership in stock in corporations traded publicly and ownership of stock through various retirement programs.