Callbacks pended by `Py_AddPendingCall` will never be invoked if the main thread doesn't release GIL

According to the Py_AddPendingCall's document (https://docs.python.org/3/c-api/init.html#c.Py_AddPendingCall), a pended callback function should be invoked when:

It will be called asynchronously with respect to normally running Python code, but with both these conditions met:

- on a bytecode boundary;

- with the main thread holding the global interpreter lock.

So if the main thread is processing an endless loop like:

while True:
    pass

A callback pended from another thread by Py_AddPendingCall should still be able to be invoked.

It works in Python 3.8.13, but doesn't work in Python 3.9.13. In 3.9.13, the callback pended by Py_AddPendingCall will never be invoked in this case.

The root cause may be here: https://github.com/python/cpython/pull/19091/files#diff-c22186367cbe20233e843261998dc027ae5f1f8c0d2e778abfa454ae74cc59deL152-L153. After this change, pending a callback from a non-main thread will not set the ceval2->eval_breaker to be true. So when the main thread is trapped in an endless loop and keeps holding GIL, pending callbacks will never be invoked.