def last_modified(self) -> str:
if "<STR_LIT>" in self.attrs:<EOL><INDENT>return self.attrs["<STR_LIT>"]<EOL><DEDENT>elif self.mode == "<STR_LIT>":<EOL><INDENT>self.attrs["<STR_LIT>"] = timestamp()<EOL>return self.attrs["<STR_LIT>"]<EOL><DEDENT>return timestamp()<EOL>
Return an ISO8601 timestamp indicating when the file was last modified

Returns:
        An ISO8601 timestamp indicating when the file was last modified

Remarks:
        If the file has no timestamp, and mode is 'r+', a new timestamp is created and returned.
        Otherwise, the current time in UTC is returned