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/contrib/jenkins.sh b/contrib/jenkins.sh
index e2e04e7..a01c012 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -18,7 +18,7 @@
 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
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