bpo-41692: Deprecate PyUnicode_InternImmortal() (GH-22486)

The PyUnicode_InternImmortal() function is now deprecated and will be
removed in Python 3.12: use PyUnicode_InternInPlace() instead.
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 500ce24..90b3299 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -261,11 +261,14 @@
     );
 
 PyAPI_FUNC(void) PyUnicode_InternInPlace(PyObject **);
-PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **);
 PyAPI_FUNC(PyObject *) PyUnicode_InternFromString(
     const char *u              /* UTF-8 encoded string */
     );
 
+// PyUnicode_InternImmortal() is deprecated since Python 3.10
+// and will be removed in Python 3.12. Use PyUnicode_InternInPlace() instead.
+Py_DEPRECATED(3.10) PyAPI_FUNC(void) PyUnicode_InternImmortal(PyObject **);
+
 /* Use only if you know it's a string */
 #define PyUnicode_CHECK_INTERNED(op) \
     (((PyASCIIObject *)(op))->state.interned)