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The differences are:

1. You're using different SSL information: You're browser has a built-in set of certificate authorities. Python uses a set which comes with the OS. They differ & if the site you're accessing uses one know to your browser but not known to python, the python will throw an exception.
2. You're accessing using different User-Agents. Your browser is telling the server it's Chrome or IE or whatever. Python is telling the server it's python. For whatever reason, the server may decide it doesn't like that and return Forbidden.
3. The server may be working harder than you think: while it appears the request is for a simple file, you're really requesting a resource. It may be (though unlikely in this case) that the resource you're requesting results in multiple interactions between the server and your browser -- cookies, javascript, etc -- which are executed successfully in your browser, returned to the server & it then delivers the file. Your python request is not doing any of that.
4. Your browser (may) have existing state which your python does not. You say you can access the file using your browser, but it could be that works only because you've accessed other resources on the site, or logged in, or whatever. Your browser is communicating that information (perhaps a session_id via cookie?) with the server recognizes. Your python code states with no previous state, so the server forbids that.

Which is it in this case? You'll need to investigate. Can you get wget or curl to work? Debug your browser's access: what headers are being sent, what are you receiving in reply?