[nl]

Python 3 had incompatible changes with Python 2. You had to update your code, and for some years it meant lots of projects stayed on Python 2.

[TylerE]

The problem was never your code, really, it was the dependencies.

Wasn't that hard for quite a while to find situations where dependency A was Py3 compatible and B was not (and the incompatibility went both ways, especially in early 3.x releases, you could NOT have one codebase that worked with both).

Sometimes A dropped Py2 support before B gained Py3.

Pain pain pain.

Then add the increasing level of insanity as the answer to "python packaging sucks" was repeatedly to add yet another layer.