_PyOS_SigintEvent() is needed for interruptible waiting on main thread

Bug report

Bug description:

Moved from https://github.com/python/cpython/pull/108605#issuecomment-2521215855

In https://github.com/python/cpython/pull/108605, _PyOS_SigintEvent was removed from the public headers.

TensorStore relies on that function here: https://github.com/google/tensorstore/blob/eda046b10158e7d82fdb0f23aeecdfdeb8ac5aa3/python/tensorstore/future.cc#L105 to enable synchronously blocking on the readiness of a custom Future type while allowing Ctrl+C to interrupt when running on the main thread.

The current workaround is to manually declare the function since the symbol is still available.

I'm not aware of a usable alternative. Since signal.set_wakeup_fd only allows a single fd to be set globally, it isn't usable from a library. The best alternative would probably be to poll by waking up every few hundred milliseconds.

CPython versions tested on:

3.13

Operating systems tested on:

No response

Start by reviewing CPython PR #108605 and the use of _PyOS_SigintEvent in TensorStore's python/tensorstore/future.cc at line 105. Trace how removing the public-header declaration affects extension libraries and compare the available signal-wakeup alternatives. Done means providing a supported way for a library to interrupt synchronous waiting on the main thread, with the behavior preserved for Ctrl+C.