input: def replace_all_occurrences(str: str, char: str, replacement: str) -> str:
      """ Replaces all occurrences of a specified character in a given string with the provided replacement character.
      >>> replace_all_occurrences("Hello World!", 'l', 'X')
      'HeXXo WorXd!'
      >>> replace_all_occurrences("Programming is fun!", 'm', 'Y')
      'PrograYing is fuY!'
      """
output:     return str.replace(char, replacement)