Perhaps done intentionally to protect the cast and crew, the snake used is actually a scarlet kingsnake which is non venomous. It differentiates itself from its venomous cousin through the tri-colored pattern. The rhyming rule "red touch yellow, kill a fellow, red touch black, venom lack" applies here. It is important to note that this rule only applies to snakes native to the US and northern Mexico.
When Donna is milking the snake, no liquid is seem streaming down from the top. But there already is liquid in the vial.