Revert "closes bpo-27494: Fix 2to3 handling of trailing comma after a generator expression (GH-3771)" (GH-8241)


This reverts commit af810b35.

This is not valid syntax (see bpo-32012).
(cherry picked from commit 4b8a7f51)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>