[downloader/http] Retry download when urlopen times out

[salty-horse]

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• Bug fix
• Improvement
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Description of your pull request and other information

When the http downloader's urlopen times out, it used to stop the program with the timeout error, disrespecting the user's retry count settings. This fix addresses that, by having a timeout trigger a retry.

I've tested it on Python 2.7.18 and 3.8.2.

While I think the fix is a useful thing to have in general, it addresses a specific issue I have, and described in #25183. That bug, however, was closed as a duplicate without any explanation or reference to a duplicate bug, so I never found out if there was any existing discussion about it.

Many times when the dash downloader is used to download youtube videos, my download would time out every 1-3 fragments. This means I would have to re-run the command over and over again.
With the timeout retry fix, the dash fragments that get a timeout are immediately retried, and the download eventually completes without any visible errors, or me having to restart it. (Note that for a smooth download I need to run it with a shortened socket-timeout, because the default of 20 seconds is too much for this particular use-case).