bpo-38644: Pass tstate explicitly in signalmodule.c (GH-19184)

PyOS_InterruptOccurred() now checks _Py_ThreadCanHandleSignals()
before checking if SIGINT is tripped.

Include/internal/pycore_pyerrors.h

@@ -65,6 +65,8 @@ PyAPI_FUNC(PyObject *) _PyErr_FormatFromCauseTstate(
     const char *format,
     ...);
 
+PyAPI_FUNC(int) _PyErr_CheckSignalsTstate(PyThreadState *tstate);
+
 #ifdef __cplusplus
 }
 #endif