structmember.h lacks a 'T_SIZET' define in order to properly support
struct fields of type 'size_t'  within PyMemberDef.

If this feature request is accepted, I can provide the patches. 

For 2.7, It would be nice to do:

#define T_SIZET      20      /* size_t */

However, this value is already used in 3.0 for 'T_NONE'. Perhaps T_NONE
should be backported to 2.7?

To avoid future merge conflicts, I would vote for this definition:

#define T_SIZET      21      /* size_t */