Modify a value in the configuration.

def set_value(self, key, value):
        # type: (str, Any) -> None
        """Modify a value in the configuration.
        """
        self._ensure_have_load_only()

        fname, parser = self._get_parser_to_modify()

        if parser is not None:
            section, name = _disassemble_key(key)

            # Modify the parser and the configuration
            if not parser.has_section(section):
                parser.add_section(section)
            parser.set(section, name, value)

        self._config[self.load_only][key] = value
        self._mark_as_modified(fname, parser)