Jake Tesler: In the meanwhile, can you please try to rewrite your change to make the attribute optional?

C PyThread_get_thread_native_id() function and Python _thread.get_native_id() should not be defined if it's not supported by 
the platform.

I suggest to add the following #define in Include/pythread.h:

#if (... list of supported platforms ...
#  define PY_HAVE_THREAD_NATIVE_ID 1
#endif

It would be great if you can come up with a solution before the end of the month, otherwise we will miss Python 3.8 deadline for new feature :-(

Tell me if you need help to rework your PR. IMHO it's a nice feature. There is just an issue in your first implementation.