build-unit-test-docker: add python3-referencing

entity-manager use python3-referencing, so add it explicitly.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I67692e95aa9e367329c48453547c315b2c5a61e5
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index a28d7e1..2ac4730 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -950,6 +950,7 @@
         isort \
         jsonschema \
         meson==1.7.0 \
+        referencing \
         requests
 
 RUN npm install -g \