input: Write a function to find pairs of numbers in a list that add up to a given sum.
  assert find_pairs([1, 2, 3, 4, 5], 8) == [(3, 5)]
  assert find_pairs([1, 2, 3, 4, 5, 6, 7], 9) == [(2, 7), (3, 6), (4, 5)]
  assert find_pairs([1, 2, 3, 4, 5, 6], 10) == [(4, 6)]
  assert find_pairs([1, 2, 3, 4, 5], 10) == []
  assert find_pairs([], 5) == []
output: def find_pairs(numbers, target_sum):
      pairs = []
      seen = set()
      for number in numbers:
          complement = target_sum - number
          if complement in seen:
              pairs.append((complement, number))
          seen.add(number)
      return pairs