input: Write a function to return unique characters from a string.
  assert unique_characters('missisippi') == {'m': True, 'i': True, 's': True, 'p': True}
  assert unique_characters('hello') == {'h': True, 'e': True, 'l': True, 'o': True}
  assert unique_characters('banana') == {'b': True, 'a': True, 'n': True}
output: def unique_characters(s):
      return {char: True for char in s}