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

contrib/jenkins.sh

@@ -18,7 +18,7 @@ fi
 virtualenv -p python3 venv --system-site-packages
 . venv/bin/activate
 pip install pytlv
-pip install pyyaml
+pip install 'pyyaml>=5.1'
 pip install cmd2==1.5
 pip install jsonpath-ng
 pip install construct