gh-71448: Support extension in BytesIO.truncate - #152252

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This PR is stale because it has been open for 90 days with no activity.

@eendebakpt could you take a look at this one?

Could you check the following cases?

data = b"abcdef"
b = io.BytesIO(data)   # buf is `data` itself (exact bytes, no copy)
b.truncate(3)          # 3 < alloc 6: quick exit, still shared
b.truncate(5)          # 5 < alloc 6: quick exit, memset
data                   # b'abc\x00\x00f'  -- an immutable constant mutated

and

b = io.BytesIO(b"abc")
b.truncate(10**9)      # Segmentation fault?

Created gh-160781 for some underlying pieces which should be backported that found while updating this

Closing until gh-160781 is done. Making this work well on large sizes is a lot easier with that. Will make a new PR after that which covers the extended cases here.

Ended up refactoring resize_buffer_lock_held into two helpers which de-duplicates code and makes the extend easier, but that needs the large size edge cases first (and pulling into this PR is a lot more work then opening a new one)

Replacement PR is up: #160933. Properly unshares branches, handles large truncates gracefully.