Here is a potential way, provided by AWS Support:

[...] Amazon RDS takes DB snapshots during the upgrade process only if you have set the backup retention period for your DB instance to a number greater than 0. [...]

Using that knowledge, one could maybe devise an upgrade strategy to set the current backups to manually taken, then reduce retention to 0, then to upgrade then to increase retention again. I have not yet tested that idea though with regards to ensuring 100% safety with regards to quick disaster recovery during the time that retention is set to 0.