Issue #13964: signal.sigtimedwait() timeout is now a float instead of a tuple

Add a private API to convert an int or float to a C timespec structure.

Include/pytime.h

@@ -3,6 +3,7 @@
 #define Py_PYTIME_H
 
 #include "pyconfig.h" /* include for defines */
+#include "object.h"
 
 /**************************************************************************
 Symbols and macros to supply platform-independent interfaces to time related
@@ -37,6 +38,16 @@ do { \
     ((tv_end.tv_sec - tv_start.tv_sec) + \
      (tv_end.tv_usec - tv_start.tv_usec) * 0.000001)
 
+#ifndef Py_LIMITED_API
+/* Convert a number of seconds, int or float, to a timespec structure.
+   nsec is always in the range [0; 999999999]. For example, -1.2 is converted
+   to (-2, 800000000). */
+PyAPI_FUNC(int) _PyTime_ObjectToTimespec(
+    PyObject *obj,
+    time_t *sec,
+    long *nsec);
+#endif
+
 /* Dummy to force linking. */
 PyAPI_FUNC(void) _PyTime_Init(void);