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> The purpose of _Py_BEGIN_SUPPRESS_IPH is to suppress such popup, no?
That macro suppresses the CRT's invalid parameter handler, which by default terminates abnormally with a fastfail (status code 0xC0000409), even in a release build. 
In a debug build, we still have message boxes from failed asserts that call _CrtDbgReportW. For example, the following stack trace shows _CrtDbgReportW called from _get_osfhandle:
 Call Site
 win32u!NtUserWaitMessage
 user32!DialogBox2
 user32!InternalDialogBox
 user32!SoftModalMessageBox
 user32!MessageBoxWorker
 user32!MessageBoxTimeoutW
 user32!MessageBoxW
 ucrtbased!__acrt_MessageBoxW
 ucrtbased!__crt_char_traits<wchar_t>::message_box<HWND__ * __ptr64,
 wchar_t const * __ptr64 const & __ptr64,
 wchar_t const * __ptr64 const & __ptr64,
 unsigned int const & __ptr64>
 ucrtbased!common_show_message_box<wchar_t>
 ucrtbased!__acrt_show_wide_message_box
 ucrtbased!common_message_window<wchar_t>
 ucrtbased!__acrt_MessageWindowW
 ucrtbased!_VCrtDbgReportW
 ucrtbased!_CrtDbgReportW
 ucrtbased!_get_osfhandle
 python310_d!PyOS_StdioReadline
_get_osfhandle validates that the fd is open via
 _VALIDATE_CLEAR_OSSERR_RETURN(_osfile(fh) & FOPEN, EBADF, -1);
which uses the following macro:
 #define _VALIDATE_CLEAR_OSSERR_RETURN(expr, errorcode, retexpr) \
 { \
 int _Expr_val=!!(expr); \
 _ASSERT_EXPR((_Expr_val), _CRT_WIDE(#expr)); \
 if (!(_Expr_val)) \
 { \
 _doserrno = 0L; \
 errno = errorcode; \
 _INVALID_PARAMETER(_CRT_WIDE(#expr)); \
 return retexpr; \
 } \
 }
In a debug build, _ASSERT_EXPR expands as:
 #define _ASSERT_EXPR(expr, msg) \
 (void)( \
 (!!(expr)) || \
 (1 != _CrtDbgReportW(_CRT_ASSERT, _CRT_WIDE(__FILE__), __LINE__, NULL, L"%ls", msg)) || \
 (_CrtDbgBreak(), 0) \
 )
which is where _CrtDbgReportW gets called. By default it reports the assertion failure with a message box. The suppression of this in libregrtest either ignores this report or sends it to stderr:
 for m in [msvcrt.CRT_WARN, msvcrt.CRT_ERROR, msvcrt.CRT_ASSERT]:
 if verbose:
 msvcrt.CrtSetReportMode(m, msvcrt.CRTDBG_MODE_FILE)
 msvcrt.CrtSetReportFile(m, msvcrt.CRTDBG_FILE_STDERR)
 else:
 msvcrt.CrtSetReportMode(m, 0)
msvcrt.CrtSetReportMode and msvcrt.CrtSetReportFile only exist in a debug build.