In my project about which I wrote in Django to the rescue I am stuck with Python 3.6.
While I have no bigger regrets about Python 3.8 as walrus operator is fine but not a killer feature for me, I was always regretful about not having dataclasses which were introduced in Python 3.7.
Yesterday, when I run pip list in my virtual environment, to my astonishment I found dataclasses in the list. Obviously, some of my libraries installed them along.
Immediately, I searched PyPI to confirm that my hunch was correct. Dataclasses has a backport for Python 3.6 here.
So if you are in a similar situation like me, you can add them to your project as an external dependency.