The cities and towns of this Commonwealth, by their duly constituted authorities, are hereby authorized annually to levy, in addition to any other levies authorized by law, a special levy upon such taxable property in such cities and towns as is not, by law, segregated to the Commonwealth for taxation, or withheld from city or town taxation, at a rate not in conflict with general law, for the purpose of providing a sinking fund or to pay the principal and interest of their bonded indebtedness, as and when the same become due and payable. All charters of any city or town are hereby repealed insofar as they are in conflict herewith.