input: Write a function to convert ratings in a list of strings to a float. The function should return a list of floats corresponding to the input list of string ratings.
  assert str_to_float(['4.5', '3.0', '2.5', '4.0'])==[4.5, 3.0, 2.5, 4.0]
  assert str_to_float(['3.3', '5.0', '4.8'])==[3.3, 5.0, 4.8]
  assert str_to_float(['2.1', '1.0', '3.9'])==[2.1, 1.0, 3.9]
  assert str_to_float([])==[]
output: def str_to_float(ratings):
      return [float(rating) for rating in ratings]