input: Write a function to find the greatest 2 numbers from a list of integers. The function should return a tuple containing the two greatest numbers in descending order.
  assert get_greatest_2([1, 5, 3, 7, 4, 7, 8, 9, 2, 10, 6, 8])==(10, 9)
  assert get_greatest_2([5, 15, 7, 3, 10, 20, 8])==(20, 15)
  assert get_greatest_2([1])==(1, float('-inf'))
  assert get_greatest_2([])==(float('-inf'), float('-inf'))
output: def get_greatest_2(lst):
      if not lst:
          return (float('-inf'), float('-inf'))
      first = second = float('-inf')
      for num in lst:
          if num > first:
              second = first
              first = num
          elif num > second:
              second = num
      return (first, second)