Just a few nitpicks on the patch (in increasing pickiness):

1. Any reason to prefer PyTuple_SetItem to PyTuple_SET_ITEM at the end of _PyLong_Divmod_Near?   You are filling a brand new tuple, so PyTuple_SET_ITEM seems to be more appropriate.

2. temp = (quo_is_neg ? long_add : long_sub)(..) is clever, but IMO is less readable than 

if (quo_is_neg)
   temp = long_add(..)
else
   temp = long_sub(..)

The later form may also be more optimization friendly, particularly if compiler wants to inline static long_add or long_sub.

3. Given that arguments are named 'a' and 'b' it is a bit confusing to have local variable c of a different type.  I think 'cmp' would be a better choice.

4. I see that you removed a comment that displays round() implemented in python.  I think it would be helpful to preserve it for documentation and testing purposes even though the actual algorithm is slightly different.  As long as the results are the same, it is helpful to have reference python code.

5. Similarly to #4, having python implementation of divmod_near() displayed somewhere will be helpful.