What kind of licensing would a person need to be able to lawfully drive a tank on city streets?

Some places, yes, some no, it depends on local law. The government usually frowns upon anything that tears up the roads and a tank is heavy and the tracks damage the road surface. Tracked vehicles are allowed on some surfaces and in some locations. Many farming places allow tracked vehicles as long as they have a rubberized surface on the tracks themselves and many in the U.S. use tracked tractors that are just as heavy as a tank. Used tanks are available for purchase in the U.S. and some are fitted with padded tracks and are legal to drive on U.S. roads. In the U.S. the tank is considered “heavy machinery”. the same as a trackhoe or tractor because the firing mechanisms in tanks guns that are sold must be removed or damaged in such a way as to make them not function.