If the temperature of the water is usable for hand dishwashing, not over 140F before mixing, and the pressure is in the normal range (40 – 60 psig) and the relief valve is still dripping, there may be dirt keeping the valve from closing against the seat. This happens because most valves sit sideways out of the body of the tank and dirt can settle against the side of the valve seat over the years. If the valve is not opened to flush the dirt periodically to keep it from accumulating, then a larger chunk can squish between the valve and the seat. This can happen even if the valve is tested often.
Flushing the valve by lifting the handle fully while a bucket is held below the pipe may clean the seat enough for it to seal. Then again it may not and the valve may require replacement. It is safer to do this after the gas has been turned off and the hot water used to cool it enough so a hand can be run under it. Then the tank only has to handle the pressure shock of popping the valve only (without a worry about scalding water.)