Reinstall signal handler when Python signal handler called

[user202729]

This would fix sagemath/sage#39601 . Not sure if it has any downside.

[user202729]

These two tests are failing: test_interrupt_bomb and test_sig_block_outside_sig_on.

The test can be fixed by adding appropriate PyErr_CheckSignals , but that feels like fixing the symptoms.

On the other hand a test could be added. It could be as simple as

import signal
from signal import SIGINT
signal.signal(SIGINT, signal.getsignal(SIGINT))