If they are that against owning a boa/python or fear them, then I suggest not to get it. You live under their roof, their money, and their rules. Say one day your boa does tag you- your parents may overreact and force you to rehome it. Or even more importantly, say that your snake gets sick and needs a few hundred dollars of vet bills... (exotic vet bills are much more expensive than dogs/cats) If you don't have the money earned yourself, your parents may not be willing to pay that amount for an animal they didn't want.
It's more fair to the snake to go to a home where there's an assurance that it will be cared for financially and likely won't be rehomed for sudden reasons.
You can still try to convince them with all the reasons you know... That you'll be the one to take care of it, bites are treated like a puncture wound and aren't venomous (lol), boas just hide all day and don't move much, they're very low maintenance animals, etc. Assuming you don't have a job and are a minor, if you can have your parents accept the new pet and they are willing to financially care for the animal, that's the only proper way to get it.