Add pip constraint for Ubuntu 18

plugins/_module/pip_update.py

@@ -40,6 +40,8 @@ def main():
         constraints['requests'] = '2.15.1'
         constraints['ruamel.yaml'] = '0.17.21'
         constraints['setuptools'] = '49.0.0'
+    if lsb_release == '18':
+        constraints['ruamel.yaml'] = '0.17.21'
     result = []
 
     changed = (lsb_release == '16') or (lsb_release == '18') or (lsb_release == '20') or (lsb_release == '22')