gh-106320: Remove private _PyInterpreterState functions (#106325)

Remove private _PyThreadState and _PyInterpreterState C API
functions: move them to the internal C API (pycore_pystate.h and
pycore_interp.h). Don't export most of these functions anymore, but
still export functions used by tests.

Remove _PyThreadState_Prealloc() and _PyThreadState_Init() from the C
API, but keep it in the stable API.

Python/compile.c

@@ -33,6 +33,7 @@
 #include "pycore_compile.h"
 #include "pycore_intrinsics.h"
 #include "pycore_long.h"          // _PyLong_GetZero()
+#include "pycore_pystate.h"       // _Py_GetConfig()
 #include "pycore_symtable.h"      // PySTEntryObject, _PyFuture_FromAST()
 
 #include "opcode_metadata.h"      // _PyOpcode_opcode_metadata, _PyOpcode_num_popped/pushed

Python/frozenmain.c

@@ -1,7 +1,8 @@
 /* Python interpreter main program for frozen scripts */
 
 #include "Python.h"
-#include "pycore_runtime.h"  // _PyRuntime_Initialize()
+#include "pycore_pystate.h"       // _Py_GetConfig()
+#include "pycore_runtime.h"       // _PyRuntime_Initialize()
 #include <locale.h>
 
 #ifdef MS_WINDOWS

Python/pystate.c

@@ -1166,7 +1166,7 @@ PyInterpreterState_GetDict(PyInterpreterState *interp)
    The GIL must be held.
   */
 
-PyInterpreterState *
+PyInterpreterState*
 PyInterpreterState_Get(void)
 {
     PyThreadState *tstate = current_fast_get(&_PyRuntime);
@@ -1408,7 +1408,7 @@ _PyThreadState_New(PyInterpreterState *interp)
 }
 
 // We keep this for stable ABI compabibility.
-PyThreadState *
+PyAPI_FUNC(PyThreadState*)
 _PyThreadState_Prealloc(PyInterpreterState *interp)
 {
     return _PyThreadState_New(interp);
@@ -1416,7 +1416,7 @@ _PyThreadState_Prealloc(PyInterpreterState *interp)
 
 // We keep this around for (accidental) stable ABI compatibility.
 // Realistically, no extensions are using it.
-void
+PyAPI_FUNC(void)
 _PyThreadState_Init(PyThreadState *tstate)
 {
     Py_FatalError("_PyThreadState_Init() is for internal use only");