Similar algorithm to apriori to find unpopular sequential patterns
The algorithms I would recommend in your case are Apriori-Inverse and Apriori-Rare. Disclaimer: I have not found a Python implementation (nor can I provide a reliable one due to time constraints), but there exists an open-source Java library SPMF with implementations of these algorithms (Inverse, Rare, and a whole bunch more) which can be used for reference -- the source code has informative comments and the algorithms are structured well enough to understand what is going on. Of course, this is only an option if you are willing to do some digging and have enough time on your hands. Maybe going through that Java code will provide you with insights on how to modify existing Python implementations to suit your needs, which should require far less effort, since Inverse and Rare differ very little from the classical Apriori. I hope this helps you at least somewhat. Good luck!