gh-109936: return on EOF in shutil.copyfileobj

[rpigott]

Alters the loop condition to break on short read.

• Issue: shutil.copyfileobj should break on short read #109936

[encukou]

As noted on the issue, a short read doesn't always indicate EOF.

[bedevere-app]

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[github-actions]

This PR is stale because it has been open for 30 days with no activity.