FWIW, I think it would make sense to keep "signals_pending" under _PyRuntimeState rather than moving it to PyInterpreterState.

Signals are only handled by the main interpreter (in its main thread).  Even though "signals_pending" is useful to only one interpreter in the runtime, making it per-interpreter sends the wrong message that it is significant at that level.  This may be confusing to readers of the code.

At the very least there should be a clear comment with the field in Include/internal/pycore_interp.h explaining how it is only used by the main interpreter and why we made it per-interpreter anyway.