Tries to extract a `datetime` object from the given string, including only hours.

        Raises `DateTimeFormatterException` if the extraction fails.

def extract_datetime_hour(cls, datetime_str):
        """
        Tries to extract a `datetime` object from the given string, including only hours.

        Raises `DateTimeFormatterException` if the extraction fails.
        """
        if not datetime_str:
            raise DateTimeFormatterException('datetime_str must a valid string')

        try:
            return cls._extract_timestamp(datetime_str, cls.DATETIME_HOUR_FORMAT)
        except (TypeError, ValueError):
            raise DateTimeFormatterException('Invalid datetime string {}.'.format(datetime_str))