Lock ruamel.yaml to 0.17.21 on Ubuntu 16

plugins/_module/pip_update.py

@@ -37,6 +37,7 @@ def main():
     if lsb_release == '16':
         constraints['borgmatic'] = '1.5.12'
         constraints['certifi'] = '2021.10.8'
+        constraints['ruamel.yaml'] = '0.17.21'
         constraints['setuptools'] = '49.0.0'
     result = []