I tend to use UnMult from Red Giant, but you can also use native tools.
Apply the Channel Combiner effect. Set its “From” property to “Max RGB” and its “To” property to “Alpha.” This will take the brightest of the three color channels and use its value for the alpha channel.
Then apply the Remove Color Matting effect (keep the default background color of black). This will eliminate black fringing in partially transparent areas.