PyNaCl is a Python binding to libsodium, which is a fork of the
Networking and Cryptography library. These libraries have a stated goal
of improving usability, security and speed. It supports Python 3.8+ as
well as PyPy 3.

python3-pynacl 1.6.0 is the last available version for Slackware 15.0.
Newer versions require cffi >= 2.0.0.