input: def replace_all_occurrences(str: str, char: str, replacement: str) -> str:
      """ Replace all occurrences of a character in a string with the provided replacement character.
      >>> replace_all_occurrences("Hello World!", 'l', 'X')
      'HeXXo WorXd!'
      >>> replace_all_occurrences("Python is fun!", 'n', 'Z')
      'PythoZ is fuZ!'
      """
output:     result = ""
      for c in str:
          if c == char:
              result += replacement
          else:
              result += c
      return result