That will be because the website is using some kind of bot detection. Someone with more time than I do will need to figure out what is needed to bypass the bot detection.
Code:
calibre-debug -c 'from calibre import browser; br = browser(); br.open("https://www.thehindu.com/todays-paper/")'
Traceback (most recent call last):
File "/usr/bin/calibre-debug", line 21, in <module>
sys.exit(main())
File "/home/kovid/work/calibre/src/calibre/debug.py", line 272, in main
exec(opts.command)
File "<string>", line 1, in <module>
File "/usr/lib/python3.10/site-packages/mechanize/_mechanize.py", line 257, in open
return self._mech_open(url_or_request, data, timeout=timeout)
File "/usr/lib/python3.10/site-packages/mechanize/_mechanize.py", line 313, in _mech_open
raise response
mechanize._response.get_seek_wrapper_class.<locals>.httperror_seek_wrapper: HTTP Error 403: Forbidden