I'm on Ubuntu 19.04 and I would like to make sure that if I try to install some software with a hard Python 2 dependency that it would just fail instead of installing 2 on my system.
See the apt-mark manpage:
PREVENT CHANGES FOR A PACKAGE
hold is used to mark a package as held back, which will prevent the package from being automatically installed, upgraded or removed.
sudo apt-mark hold python
Note that this is a Really Bad Idea on Ubuntu 16.04 and earlier, as Python 2 is an essential part of those releases.