bpo-42643: Add support for HTTP range requests

[DavidBord]

https://bugs.python.org/issue42643

[github-actions]

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

[roolebo]

@DavidBord Thanks! I think built-in range support is going to be helpful

[stokito]

It’s needed to resume downloads and this may significantly reduce load in some cases. That’s why even minimal busybox httpd has a support of Range requests

[github-actions]

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

[chrisroat]

This is a most welcome update. Thank you @DavidBord

[imba-tjd]

Personally I think it's fundamental to support Range: bytes=<range-start>- i.e. no <range-end>.

Others I think are OK.

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I'm using a modified version of this patch in my program https://github.com/imba-tjd/qrsend. I will share info when I found defects.

[imba-tjd]

Fatal error. When you use 2-5, the start_byte is 2 and end_byte is 5. Then you read(5).
The correct behavior is read(4), which is end_byte-start_byte+1. Though I won't suggest whether putting +1 here or previous place.

More importantly, read() will load all data into memory in a row before start writing. This has serious performance issue when resuming large file. I tried source.truncate() but it turns out an io.UnsupportedOperation.

Besides Content-Length needs changing too. So that new testing is recommended to be added, too.