Make `os.scandir` retry on system calls failing with EINTR

Bug report

Bug description:

In a long-running script, I hit:

[…]
  File "/Users/gsnedders/ct.py", line 104, in worker
    with os.scandir(path) as it:
         ~~~~~~~~~~^^^^^^
InterruptedError: [Errno 4] Interrupted system call: '/Users/gsnedders/Library/Containers/com.apple.ScreenTimeWidgetApplication.ScreenTimeWidgetExtension/Data/Library/Application Scripts'

No repro case is provided, because that would require making syscalls fail with EINTR reliably somehow.

PEP 475 (Retry system calls failing with EINTR) implies to me that os.scandir should be retrying, rather than just failing here.

It does appear that os.scandir doesn't always retry, looking at the source:

https://github.com/python/cpython/blob/ae3064608935367c860182dc1b364631082ecdff/Modules/posixmodule.c#L16489

The iterator itself also seems to not retry:

https://github.com/python/cpython/blob/ae3064608935367c860182dc1b364631082ecdff/Modules/posixmodule.c#L16290

CPython versions tested on:

3.13

Operating systems tested on:

macOS

PEP 475 と、Modules/posixmodule.c にあるリンク先の os.scandir 実装（参照されている行のイテレーターコードを含む）を読みます。EINTR を返すシステムコールがどのように処理されるかを確認し、os.scandir が InterruptedError を発生させるのではなく再試行することを検証します。初回の呼び出しとイテレーターの動作の両方が意図した再試行ポリシーに一致すれば完了です。