https://github.com/python/cpython/pull/7043/commits/053111f321d792fb26f8be10abba24a980f3590f

I added one micro optimization.
Although it is not relating to two-word-gc directly, it makes gc_collect faster than master on the microbench (without importing tkinter.tix, because no GUI on my Linux environment).

$ ./python -m perf timeit --compare-to ./python-master -s "import gc, doctest, ftplib, asyncio, email, http.client, pydoc, pdb, fractions, decimal, difflib, textwrap, statistics, shutil, shelve, lzma, concurrent.futures, telnetlib, smtpd, trace, distutils, pkgutil, tabnanny, pickletools, dis, argparse" "gc.collect()"
python-master: ..................... 1.66 ms +- 0.08 ms
python: ..................... 1.58 ms +- 0.00 ms

Mean +- std dev: [python-master] 1.66 ms +- 0.08 ms -> [python] 1.58 ms +- 0.00 ms: 1.05x faster (-5%)