input: Write a function to calculate the average of a list of numbers in Python.
  assert avg_list([1, 2, 3, 4, 5])==3
  assert avg_list([10, 20, 30, 40, 50])==30
  assert avg_list([])==None
  assert avg_list([5])==5
  assert avg_list([7, 14, 21, 28, 35])==21
  assert avg_list([10, 10, 10, 10, 10])==10
output: def avg_list(lst):
      if not lst:
          return None
      return sum(lst) / len(lst)