Here's a patch to add expm1.  Rather than putting the code in pymath.c, 
which is the wrong place (see issue #7518), I've added a new file 
Modules/_math.c for this;  log1p, atanh, etc. should also eventually be 
moved from pymath.c into this file.

I'm fairly confident that the maths and numerics are okay, but less 
confident about my changes to the build structure;  if anyone has a chance 
to test this patch, especially on Windows, I'd welcome feedback.  I've 
tested it on OS X, and will test on Linux.