Without actually looking, I am skeptical of _decimal.c as I expect the code to be highly technical. But maybe it has more helpful comments than I expect. But I am willing to let the person pushing decide. I am overall in favor of linking to python sources.

difflib I know should be linked. tokenize.py (not in the patch) probably should be linked eventually, but only after the docstrings (and doc) are fixed a bit. I will think about it after that is done.