input: Write a function to return the maximum number in a list.
  
  assert find_max([1, 15, 20, 6])==20
  assert find_max([-1, -15, -20, -6])==-1
  assert find_max([3])==3
  assert find_max([0, 0, 0, 0])==0
  assert find_max([10, 20, 30, 40, 50])==50
output: def find_max(lst):
      return max(lst)