gh-128844: Make _Py_TryIncref public as an unstable API. (#128926)

This exposes `_Py_TryIncref` as `PyUnstable_TryIncref()` and the helper
function `_PyObject_SetMaybeWeakref` as `PyUnstable_EnableTryIncRef`.

These are helpers for dealing with unowned references in a safe way,
particularly in the free threading build.

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Objects/object.c

@@ -2588,6 +2588,20 @@ PyUnstable_Object_EnableDeferredRefcount(PyObject *op)
 #endif
 }
 
+int
+PyUnstable_TryIncRef(PyObject *op)
+{
+    return _Py_TryIncref(op);
+}
+
+void
+PyUnstable_EnableTryIncRef(PyObject *op)
+{
+#ifdef Py_GIL_DISABLED
+    _PyObject_SetMaybeWeakref(op);
+#endif
+}
+
 void
 _Py_ResurrectReference(PyObject *op)
 {