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I found that http://en.wikipedia.org/robots.txt returns 403 if the provided user agent is in a specific blacklist.
And since robotparser doesn't provide a mechanism to change the default user agent used by the opener, it becomes unusable for that site (and sites that have a similar policy).
I think the user should have the possibility to set a specific user agent string, to better identify their bot.
I attach a patch that allows the user to change the opener used by RobotFileParser, in case the need of some specific behavior arises.
I also attach a simple example of how it solves the issue, at least with wikipedia.