bpo-39947: Add PyInterpreterState_Get() function (GH-18979)

* Rename _PyInterpreterState_Get() to PyInterpreterState_Get() and
  move it the limited C API.
* Add _PyInterpreterState_Get() alias to PyInterpreterState_Get() for
  backward compatibility with Python 3.8.
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index fbb0899..96df0d9 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -139,13 +139,8 @@
 
 };
 
-/* Get the current interpreter state.
-
-   Issue a fatal error if there no current Python thread state or no current
-   interpreter. It cannot return NULL.
-
-   The caller must hold the GIL.*/
-PyAPI_FUNC(PyInterpreterState *) _PyInterpreterState_Get(void);
+// Alias for backward compatibility with Python 3.8
+#define _PyInterpreterState_Get PyInterpreterState_Get
 
 PyAPI_FUNC(PyThreadState *) _PyThreadState_Prealloc(PyInterpreterState *);