gh-106320: Remove private _PyOS_IsMainThread() function (#108605)

Move the following private API to the internal C API
(pycore_signal.h): _PyOS_IsMainThread() and _PyOS_SigintEvent().

Author: vstinner

Include/internal/pycore_signal.h

@@ -95,6 +95,15 @@ struct _signals_runtime_state {
     }
 
 
+// Export for '_multiprocessing' shared extension
+PyAPI_FUNC(int) _PyOS_IsMainThread(void);
+
+#ifdef MS_WINDOWS
+// <windows.h> is not included by Python.h so use void* instead of HANDLE.
+// Export for '_multiprocessing' shared extension
+PyAPI_FUNC(void*) _PyOS_SigintEvent(void);
+#endif
+
 #ifdef __cplusplus
 }
 #endif

Include/intrcheck.h

@@ -5,25 +5,18 @@ extern "C" {
 #endif
 
 PyAPI_FUNC(int) PyOS_InterruptOccurred(void);
+
 #ifdef HAVE_FORK
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03070000
 PyAPI_FUNC(void) PyOS_BeforeFork(void);
 PyAPI_FUNC(void) PyOS_AfterFork_Parent(void);
 PyAPI_FUNC(void) PyOS_AfterFork_Child(void);
 #endif
 #endif
+
 /* Deprecated, please use PyOS_AfterFork_Child() instead */
 Py_DEPRECATED(3.7) PyAPI_FUNC(void) PyOS_AfterFork(void);
 
-#ifndef Py_LIMITED_API
-PyAPI_FUNC(int) _PyOS_IsMainThread(void);
-
-#ifdef MS_WINDOWS
-/* windows.h is not included by Python.h so use void* instead of HANDLE */
-PyAPI_FUNC(void*) _PyOS_SigintEvent(void);
-#endif
-#endif /* !Py_LIMITED_API */
-
 #ifdef __cplusplus
 }
 #endif