input: Write a function to classify letters in a string into uppercase and lowercase.
  assert classify_letter_types('HelloWorld') == {'lowercase_letters': ['e', 'l', 'l', 'o', 'o', 'r', 'l', 'd'], 'uppercase_letters': ['H', 'W']}
  assert classify_letter_types('PythonProgramming') == {'lowercase_letters': ['y', 't', 'h', 'o', 'n', 'r', 'o', 'g', 'r', 'a', 'm', 'm', 'i', 'n', 'g'], 'uppercase_letters': ['P', 'P']}
  assert classify_letter_types('DataScience') == {'lowercase_letters': ['a', 't', 'a', 'c', 'i', 'e', 'n', 'c', 'e'], 'uppercase_letters': ['D', 'S']}
output: def classify_letter_types(s):
      result = {'lowercase_letters': [], 'uppercase_letters': []}
      for char in s:
          if char.islower():
              result['lowercase_letters'].append(char)
          elif char.isupper():
              result['uppercase_letters'].append(char)
      return result