scripts: west sign: fix devicetree module include

PR #33746 introduced changes to the devicetree python file
that requires changes in the python code that imports the
devicetree module.

This was omitted in the west sign command implementation.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@prevas.dk>
diff --git a/scripts/west_commands/sign.py b/scripts/west_commands/sign.py
index 8a0fd7d..dce7fb7 100644
--- a/scripts/west_commands/sign.py
+++ b/scripts/west_commands/sign.py
@@ -23,7 +23,7 @@
     ZEPHYR_SCRIPTS
 
 # This is needed to load edt.pickle files.
-sys.path.append(str(ZEPHYR_SCRIPTS / 'dts'))
+sys.path.append(str(ZEPHYR_SCRIPTS / 'dts' / 'python-devicetree' / 'src'))
 
 SIGN_DESCRIPTION = '''\
 This command automates some of the drudgery of creating signed Zephyr