Nonsense. Use six, use tox for testing in both, easy peasy. They're not that different. Plenty of libraries do it. No reason scripts can't too. Plenty do. Specify a "/usr/bin/env python3" shebang if you want to use any cool features like f-strings
tox apparently depends on virtualenv; this is reasonable if your software is primarily Python, but absurd if your package merely uses Python for one-off scripts. macOS doesn't even install pip by default.
I guess what I'm saying is that Python 2 had an opportunity to fill the Perl/sh ubiquity niche (installed by default on Linux/macOS/FreeBSD!), but squandered it with the 2->3 incompatibilities.
You can writes compatible Python code without using any extensions, I did that myself, but those tools just make it easier.
> I guess what I'm saying is that Python 2 had an opportunity to fill the Perl/sh ubiquity niche (installed by default on Linux/macOS/FreeBSD!), but squandered it with the 2->3 incompatibilities.
Since you mentioned FreeBSD I need to add that while ago they had an effort to remove any scripting language from the base system. So FreeBSD doesn't come with Perl, Python, Ruby etc preinstalled.
Actually, I think that was a really good decision. No need to worry that system depends on some old Python module which if you upgrade, you can break it.
Instead, you need python3.6? You install that version, you realize that something else needs python2.7? You install python2.7 side by side. In fact you can install all available versions without conflicts or worrying that something will break.
[1] https://wiki.python.org/moin/PortingToPy3k/BilingualQuickRef