input: Write a function to calculate the average of a list of numbers. If the list is empty, return None.
  def avg_list(lst):
      if len(lst) == 0:
          return None
      sum = 0
      count = len(lst)
      for i in lst:
          sum += i
      avg = sum / count
      return avg
  
  assert avg_list([1, 2, 3, 4, 5]) == 3.0
  assert avg_list([10, 20, 30, 40, 50]) == 30.0
  assert avg_list([0, 0, 0]) == 0.0
  assert avg_list([]) == None
  assert avg_list([7]) == 7.0
output: def avg_list(lst):
      if len(lst) == 0:
          return None
      sum = 0
      count = len(lst)
      for i in lst:
          sum += i
      avg = sum / count
      return avg