Alternately, you could just update python3-gmpy2 to the latest version of the package, which is built to support python3.8:
https://packages.debian.org/sid/amd64/python3-gmpy2/filelist Python3.7 is no longer supported in Debian Unstable and Testing and will be removed shortly. What you are observing is a perfectly normal transition to a newer version of python3. If such things bother you this much, you should probably stick to a Debian Stable release. This is the 7th time we've done this for Python3. It happens once or twice a release cycle. There's no bug at all here. Scott K
