On Linux, the following C program tells me that pthread_t is unsigned.
---
#include <pthread.h>
#include <stdio.h>

#define TYPE_IS_SIGNED(TYPE) ((TYPE)-1 < (TYPE)0)

int main()
{
    printf("signed? %i\n", TYPE_IS_SIGNED(pthread_t));
    return 0;
}
---

So it's fair to modify threading.get_ident() to return an unsigned number.

But I disagree to change stable Python versions, it may break applications.

Oh, I wrote write_thread_id() in Python/traceback.c and this function already casts the thread identifier to an unsigned number ;-)