> Classifiers were always documented as lists (msg214915) and passing a non-list type was raised a cryptic exception message as already reported in my first message

That doesn't matter.  You can't break a bunch of packages in a 3.x release with no warning just because people are doing something against what the documentation says.  That's not how we develop Python. How is a user of a 3rd party package going to fix this?  They have to ask the 3rd party package author to fix their setup.py code.  Until then, they cannot use Python 3.7.  This patch needs to be reverted and reworked, IMHO.

I trying installing the top packages listed on https://pythonwheels.com .  A number of them fail because of this change, see attached text file.