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There's lots of specialized tools for various through-hole components overall, all of them generally called "lead benders". For TO-220 it appears that the term is _lead bending/forming pliers_. And there exist lots of different versions, depending on how you want the legs to go. 

This site (I'm not affiliated) has some nice illustrations of different examples: https://www.piergiacomi.com/piergiacomi/en/products/hand-tools/379-preformatori-to-dettaglio.html

That's for professional assembly lines. Otherwise, the general flat nose pliers that you use for most through-hole work will suffice for low volumes and prototype work.