Given your parameters, I highly suggest a cat, fish or both. When programming, I've found that my companion of a cat (two tiger striped male and female, 6 yrs and 4 yrs old) are fantastic mood enhancers, that is when they're not crying like babies for food. Barely any maintenance when compared to a dog, but the only downside for me personally is cleaning the litter box (could always get the automatic litter box that cleans itself), but it's extremely necessary if you expect your pet to be clean, which translates to a more hygienic living space for you too.

Fish are also great mood enhancers but can be a bit more work in my opinion than cats, at least for the initial setup and routine tank cleaning. But overall - just sitting back, reflecting on some code while staring at the fish tank has worked wonders for me producing small epiphanies.