I am sorry this snowballed.  The intent in my messages here and in my PR review was to exchange a late, unfriendly traceback with a clear, early message, but only for package authors.  I forgot that a Python 3.7 could execute an invalid setup.py from an existing tarball, as Neil pointed with the pip install exifread example.  Even if these packages get PRs to build wheels, it’s still bad to break existing sdists.  +1 to reverting the patch, +1 to reverting the breakage and also fixing the original problem with a warning and explicit conversion.