input: Write a function to find common characters in two strings.
  assert find_common_characters("hello", "world") == ['o', 'l']
  assert find_common_characters("python", "java") == []
  assert find_common_characters("apple", "grape") == ['a', 'p', 'e']
output: def find_common_characters(str1, str2):
      return list(set(str1) & set(str2))