That is a baby ratsnake (Pantherophis obsoletus). Completely harmless rodent eaters.

Even more disturbing is that there are "professional" snake removal "experts" who recommend and use these awful traps as a "humane" alternative to killing snakes! What's humane about what you see above, please tell me? Even if the snake is found and released, a tedious process involving pouring oil all over the animal and waiting for it to break the connection of the glue, very often the snake has already been injured internally if not visibly, and now it's nostrils are clogged with thick oil. Its release usually just means it will go off and die elsewhere. Due to their metabolism, reptiles die much more slowly and feel pain for a longer period of time than a mammal or bird exposed to the same sort of abuse. No animal, not even a pest rodent species, deserves this kind of torture.

I assume that is a captive snake (or an escaped captive). While the pattern and coloration might first suggest that it is a striped morph California Kingsnake (Lampropeltis getula), the head shape and pattern tells a different story. The head shape looks to me like that of a milksnake (Lampropeltis triangulum). Therefore I am pretty sure this is one of the captive hybrids between a Cal King and a Milksnake.

The only poisonous snakes in Wisconsin will have a rattle at the end of their tails. Also, those are very very rare here. Also, snakes are your friends. They will keep rodent populations under control. They are generally very shy and won't bother you unless messed with, or stepped on (do not try to pick up).