Merge "update cliff to 3.5.0 support python3.8"

Author: Zuul

lower-constraints.txt

@@ -2,7 +2,7 @@ alabaster==0.7.10
 appdirs==1.3.0
 asn1crypto==0.23.0
 cffi==1.14.0
-cliff==2.8.0
+cliff==3.5.0
 cmd2==0.8.0
 coverage==4.0
 cryptography==2.7