Welp, Python+numerics/stats stack is really messy; new stuff like coroutines can be a mindfuck. But what of what was once easy in Python is now harder?
Parentheses after print statements?
Parentheses after print statements?
If you carve out the Python 2.0-ish subset and taught a class on that, it would be as easy as it ever was. But if your students take that knowledge and go to the Django tutorial, they're going to find themselves missing several concepts, and Django doesn't even use everything in Python, being rather old itself.
That said, I still think I'd rather take a Python 2.0-ish subset and teach students that, than give them some new custom language that they can't expand on or use in the field. Python 2.0-ish is still pretty useful and there's a clean upgrade track if you do want to continue down that road. And even the latest Python is still, at worst, middle-of-the-pack on language difficulty, if not still trending towards the easy side in the list of top 10-20 languages. But it's definitely a non-trivial upgrade path now.
I would add, "finding a third-part library", but that was mostly because there were not as many. :) Then again the current glut of libraries is a bit of mixed blessing. I can look for a library that does X and find dozens of them. Very few of them will be mature or do what I want. But that has more to do with the ecosystem than with Python-the-language.