input: Write a function to convert a list of string ratings to a list of floats. 
  
  assert str_to_float(['4.5', '3.0', '2.5', '4.0']) == [4.5, 3.0, 2.5, 4.0]
  assert str_to_float(['5.0', '2.1', '4.7']) == [5.0, 2.1, 4.7]
  assert str_to_float([]) == []
  assert str_to_float(['1.0', '0.5', '3.5']) == [1.0, 0.5, 3.5]
output: def str_to_float(ratings):
      return [float(rating) for rating in ratings]