The maximum amount of privilege or license tax which the several municipalities within the state may annually assess and collect of persons operating electric or hydroelectric street railroads, electric light and power companies, gas companies, waterworks companies, pipe line companies for transporting or carrying gas, oil, gasoline, water or other commodities, gas distributing companies, whether by means of pipe lines or by tanks, drums, tubes, cylinders, or otherwise, heating companies or other public utilities, incorporated under the laws of this state or any other state or whether incorporated at all or not, except telephone and telegraph companies, railroad and sleeping car companies and express companies which are otherwise licensed shall not exceed three percent of the gross receipts of the business done by the utility in the municipality during the preceding year; and, for the first year’s business when an existing utility is taken over, the amount of the license shall be computed on the basis of the gross receipts of the prior operators plus the gross receipts of the new owners; provided, that this section shall not affect any existing contract between any municipality and any public utility operating therein, except those provisions of contracts which relate to the amount or basis of the license tax imposed by such municipality on such utility.