Yes, 403 means forbidden, probably because the request has no acceptable user-agent.
Don't forget that each file reading costs money to the site, so they avoid automated downloads.
Imagine: if you run your sketch dozens of times to test it, that's a download each time (while a browser might just cache the file), at the end it is costly...
It is simpler (and faster) to download the file once and to read it from the disk.

Yes is true:  http://www.gutenberg.org/error403.php

Your code is right just try testing it with another file.