[3.11] gh-119506: fix _io.TextIOWrapper.write() write during flush (#119507)

[methane]

This PR is backport of #119507

This PR fixes TextIOWrapper's inner buffer breakage.
The bug caused assertion error when Python debug build.
I am not sure it can cause crash on release build. The bug will cause:

• Lost buffered data, and,
• NUL bytes can be written instead of the lost data.

Since this bug is not verified to cause crash (and DoS) in release build, I am not sure this PR is merged into security fix branches.
But this bug can cause serious bug (data lost/breakage). So I created this PR before details went away from my head.

• Issue: _io.TextIOWrapper.write: write during flush causes pending_bytes length mismatch #119506

[hugovk]

@pablogsal Can this be merged into 3.11 or 3.10?

[pablogsal]

I think it makes sense to back port although doesn't seem too critical IMHO

[terryjreedy]

Only 'special' people can merge into 3.11 and before.
@pablogsal Can you still do so as release manager? @ambv I know you can. Pablo approved.
Please one of you merge or close this orphaned backport.

[hugovk]

I can do it, let's update the PR from 3.11 first to be sure the CI still passes.