update pyyaml dependency to >= 5.1

5.1 was the version introducing pyyaml.FullLoader which we're using,
see https://pyyaml.org/wiki/PyYAML#history

Change-Id: I0f2fa08ceeac2759218e85ad5bdce3ef951d0b74
diff --git a/requirements.txt b/requirements.txt
index 7308cf8..0046a50 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,4 +6,4 @@
 construct
 bidict
 gsm0338
-pyyaml
+pyyaml>=5.1