add python requirement ruamel.yaml.clib==0.2.2 to keep python 2.7 compatible (#7754)

requirements.txt

@@ -5,4 +5,5 @@ netaddr==0.7.19
 pbr==5.4.4
 jmespath==0.9.5
 ruamel.yaml==0.16.10
+ruamel.yaml.clib==0.2.2
 MarkupSafe==1.1.1