input: Write a python function to find unique characters in a string.
  assert unique_chars("hello")=={'h', 'e', 'l', 'o'}
  assert unique_chars("world")=={'w', 'o', 'r', 'l', 'd'}
  assert unique_chars("python")=={'p', 'y', 't', 'h', 'o', 'n'}
output: def unique_chars(s):
      return set(s)