scripts/dts: Fix for PyYAML 5.1 Loader changes

PyYAML 5.1 was just released and it doesn't support !include as
previous versions do.  This breaks our DTS bindings parsing.

Let's fix our extract_dts_include.py script to work with both
3.13 and 5.1.

Also, update the pyyaml requirement to >=3.13 to be sure we're
compatible.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/14496



Signed-off-by: Michael Scott <mike@foundries.io>