I think pypy might be a simple way to get around that, iirc it's pretty self-contained and doesn't need to integrate much. I ran it from /opt for a while, but ~/.local should work too.
Did they? There's no /usr/bin/python any more in testing/unstable since last week, but python2 is still there and I don't think it's going away very soon. Even if it did, you can always grab python2 from stable.
Simply git clone https://github.com/apprenticeharper/DeDRM_tools/ and then:
or (I'm not exactly sure where to get adeptkey.der but I'm certain it can be extracted from an old python2 Calibre + DeDRM installation.)