Forget python development, why is it so damn hard to USE python scripts in a version-correct way in the first place? I have python scripts in my system which re

Forget python development, why is it so damn hard to USE python scripts in a version-correct way in the first place? I have python scripts in my system which rely on different python versions. And since I have a default python version, 2.7 (through symlink or env or whatever), all the python 3 scripts fail until I switch the default version manually (due to library dependencies, print format, etc.). Why don't these scripts add "#!/usr/bin/python3" at the top? And why is it so hard to just have 2 versions co-exist? I MUST be doing something wrong here? EDIT: "python3 <script.py>" doesn't always work because some scripts are written in bash and they call python within the script.