input: Write a function to identify unique characters in a string.
  assert identify_characters('Abe erotically jitters') == ['A', 'b', 'e', ' ', 'r', 'o', 't', 'i', 'c', 'a', 'l', 'y', 'j', 'i', 't', 's']
  assert identify_characters('hello') == ['h', 'e', 'l', 'o']
  assert identify_characters('Python programming') == ['P', 'y', 't', 'h', 'o', 'n', ' ', 'p', 'r', 'g', 'a', 'm', 'i', 'n']
output: def identify_characters(s):
      seen = set()
      unique_chars = []
      for char in s:
          if char not in seen:
              unique_chars.append(char)
              seen.add(char)
      return unique_chars