extract_dts_includes.py: deterministic flash table with Python < 3.6
extract_dts_includes.py: deterministic flash table with Python < 3.6

To reproduce the issue this addresses, run "cmake -B96b_nitrogen"
twice with Python 3.5. Observe the order of the flash partition table
changing in:
"build{1,2}/zephyr/include/generated/generated_dts_board.conf" and
"generated_dts_board_unfixed.h"

Dictionaries are iterated in a random order by Python 3.5 and before.
This could have caused "Unstable" CI in PR #13921 and maybe others.
Anyway we want builds to be determimistic by default. Explicit
randomness can be added for better coverage but not by default.

Signed-off-by: Marc Herbert <77e7c7d746b52c6eb01119b65a031ae1f650246d@intel.com>

GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr