Fix 'I/O operation on closed file' and 'Form data has been processed already' upon redirect on multipart data

[GLGDLY]

What do these changes do?

Fix 'I/O operation on closed file' and 'Form data has been processed already' upon redirect on multipart data, based on the discussion in #6853 .

This approach tries to pre-build Payload object for the data passing into the request, before the redirect while True loop starts, so we can reuse the same Payload object for the entire redirect chain. However, it is notable that I/O object is always an issue, so my thought here is to use the seek operation on the I/O object to allow chunk writing on redirect requests.

Yet, for non-seekable I/O objects, some possible solution are:

• sending expect 100 first
• local buffering
• raise error

I think more discussion might be needed for non-seekable objects, so I just raise error in this PR first.

Another thing I think worth discussion is that I removed the close() operation on the I/O object in this PR due to the following reasons:

• StringIOPayload do not close its I/O value in the original code,
• Closing of the I/O value will make the payload un-reusable, even if it is seekable
• We are not the one to create the I/O value, so maybe we should leave for user to close it.
• standard I/O object will be closed upon destructure, so it won't affect the user interface

But I do think more discussions might be needed here.

Are there changes in behavior for the user?

No.

Is it a substantial burden for the maintainers to support this?

Yes.

Related issue number

Fixes #5577 Fixes #5530

Checklist

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