No data available for latitude greater than N 60°

Understood that NASE JPL provides only data up N 60° latitude due to https://www.usna.edu/Users/oceano/pguth/md_help/html/srtm.htm. But, there are other sources like http://viewfinderpanoramas.org/dem3.html providing SRTM data files for more nothern countries.
Those data files are the same SRTM3 format and due to my findings they are compatible with srtm.py - at least up to N 60°. I just had to place them in the same caching directory like the other downloaded files.
However, copying those SRTM files for higher latitude than N °60 does not work that way.
I am always receiving the response "None".
What could be done?

Just to add a fast solution to the issue I mentioned: you can overcome the issue by adding the additional HGT files (for more N 60°) not only to the related directory but also to the file list.json!
Due to those findings my proposal would be that the library looks first to the cached files stored locally and afterwards to list.json in order to place the server request...