What a clever idea!
I think this one will ultimately fall to the DM to decide/mediate.
The Conjure Animals spell states that the animals you conjure disappear. If you extract venom from the animal, it is arguably neither the animal itself nor a part of it and therefore not necessarily subject to disappearing with the snake itself.
Serpent Venom is an injury poison:
Injury poison can be applied to weapons, ammunition, trap components, and other objects that deal piercing or slashing damage and remains potent until delivered through a wound or washed off.
This means it is good for one use before needing to be reapplied which limits its usefulness in combat to one attack. That's hardly game breaking in my eyes.
As a DM, I personally would handle this matter by requiring the player to make a skill check to harvest the venom per the DMG:
A character can instead attempt to harvest poison from a poisonous creature, such as a snake, wyvern, or carrion crawler. The creature must be incapacitated or dead, and the harvesting requires 1d6 minutes followed by a DC 20 Intelligence (Nature) check. (Proficiency with the poisoner's kit applies to this check if the character doesn't have proficiency in Nature.)
To me, burning a 3rd level spell slot to summon the snakes and spending the time to harvest the venom seem like more than sufficient costs* compared to the 3d6 bonus damage it can do for a single attack, even if multiple samples of the poison are harvested and shared between the party members.
Upper_Case points out some other considerations that bear thinking about as a DM. If the party tries to sell the venom, valued at 200gp, that changes the balance a bit and I agree with him that this would be exploitative and would either prevent its sale or homebrew a different, much lower value.
If the party decides to stockpile venom or take advantage of this lenient ruling, a discussion might need to be had (although I will point out that poison is one of the most commonly resisted/immune damage types, so it may not be as effective as the party hopes).