Fix module immutabledict not found

PiperOrigin-RevId: 602948518

requirements.txt

diff --git a/requirements.txt b/requirements.txt
index 3cf63bf..37656a8 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,4 @@
 mobly==1.12
 inflection
 absl-py
+immutabledict