gh-106320: Remove private pythonrun API (#108599)

Remove these private functions from the public C API:

* _PyRun_AnyFileObject()
* _PyRun_InteractiveLoopObject()
* _PyRun_SimpleFileObject()
* _Py_SourceAsString()

Move them to the internal C API: add a new pycore_pythonrun.h header
file. No longer export these functions.

Include/internal/pycore_pythonrun.h

@@ -0,0 +1,39 @@
+#ifndef Py_INTERNAL_PYTHONRUN_H
+#define Py_INTERNAL_PYTHONRUN_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+extern int _PyRun_SimpleFileObject(
+    FILE *fp,
+    PyObject *filename,
+    int closeit,
+    PyCompilerFlags *flags);
+
+extern int _PyRun_AnyFileObject(
+    FILE *fp,
+    PyObject *filename,
+    int closeit,
+    PyCompilerFlags *flags);
+
+extern int _PyRun_InteractiveLoopObject(
+    FILE *fp,
+    PyObject *filename,
+    PyCompilerFlags *flags);
+
+extern const char* _Py_SourceAsString(
+    PyObject *cmd,
+    const char *funcname,
+    const char *what,
+    PyCompilerFlags *cf,
+    PyObject **cmd_copy);
+
+#ifdef __cplusplus
+}
+#endif
+#endif  // !Py_INTERNAL_PYTHONRUN_H
+