Note that on Linux you may have existing options for running on python3:
If you are using Arch Linux, I'm the distro maintainer there, and for a couple of releases I've provided split packages for "calibre" and "calibre-python3". You can toggle between the two, if you have both installed, using the helper command
Code:
calibre-alternatives
Fedora exclusively provides a python3 build of calibre due to the deprecation of python2 itself.