input: Write a python function to find pairs of numbers that sum to zero in a list and return the count of such pairs.
  assert pairs_sum_to_zero([1, -1, 2, 3, -2, -3]) == 3
  assert pairs_sum_to_zero([4, -4, 5, 6, -5, -6, 0]) == 3
  assert pairs_sum_to_zero([1, 2, 3, 4, 5]) == 0
output: def pairs_sum_to_zero(lst):
      count = 0
      seen = set()
      for num in lst:
          if -num in seen:
              count += 1
          seen.add(num)
      return count