BytesIO and StringIO issues with large sizes #160781

Bug report

1. BytesIO and StringIO .truncate reject sizes above 2**31 - 1 on Windows. This was introduced in a move from Argument Clinic converters to hand conversion (BytesIO in Segfault/abort from calling BytesIO unshare_buffer in threads on a free-threaded build #132551, StringIO in fix thread safety of io.StringIO.truncate #133732).

2. BytesIO.write near sys.maxsize raises SystemError instead of OverflowError.

   import io, sys
   b = io.BytesIO(b"abc")
   b.seek(sys.maxsize - 1)
   b.write(b"x")

Found while working on gh-71448

CPython versions tested on:

CPython main branch, 3.14, 3.15

Operating systems tested on:

Linux, Windows

Linked PRs

• gh-160781: Fix BytesIO overflow handling for large sizes #160783
• gh-160781: Fix StringIO.truncate() rejecting sizes above 2**31 - 1 on Windows #160784