[fetchart] Add retry with exponential backoff

[richardmitic]

Description

The iTunes source returns 403 as a basic form of rate limiting. To get around this, we add a retry with exponential backoff to all requests made by _logged_get(). We also include some other generic error codes to retry.

Fixes #3827, which was closed with no fix. Without this change, two different runs of beet fetchart -f will download different sets of album art because the iTunes has art available but will fail some requests with 403.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

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[ybnd]

Fixes the issue as advertised, nice work!
However, a recent breaking change in urllib3 makes it so we have to jump through some hoops to get it to work.

Tested by running beet fetchart over a large library with itunes as the only source

• Before this PR, the logs showed multiple requests failing with HTTP 403 like so

     fetchart: trying source itunes for album Chromatics - In the City
     fetchart: getting URL: https://itunes.apple.com/search?term=Chromatics+In+the+City&entity=album&media=music&limit=200
     fetchart: iTunes search failed: 403 Client Error: Forbidden for url: https://itunes.apple.com/search?term=Chromatics+In+the+City&entity=album&media=music&limit=200
     fetchart: Chromatics - In the City: no art found
  

• With this PR, those failing requests do not appear anymore -- instead the process pauses once in a while before carrying on.

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[richardmitic]

@ybnd Rebased on master and removed the contentious method_whitelist/allowed_methods keyword. What do you think?

[ybnd]

@richardmitic looks good!
I'll merge as soon as I get the chance to run this "live" again to confirm.

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.