Add pyyaml to requirements.txt

Change-Id: I3430c32aea59af97360b9e766bfe95a146f09fe0
diff --git a/requirements.txt b/requirements.txt
index b94bf24..7308cf8 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,3 +6,4 @@
 construct
 bidict
 gsm0338
+pyyaml