CLN: Replaced package list with find_packages
- [X] closes #18535 
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

As far as local testing goes, I built source distributions using py27 and py36. Compared to HEAD, the generated distributions were practically identical. Technically they were off by a size of around 100 bytes, but I was getting that same variation when generating clean built distributions just on HEAD alone. Running a recursive diff showed some slight differences in the ``.so`` files, but given I saw that variation without making any changes I think it is OK.