dist/tools/backport_pr: use f-strings where possible
dist/tools/backport_pr: use f-strings where possible

miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT