Bump lower-constraint for decorator to 4.2.1

This patch bumps the lower-constraint for decorator to 4.2.1
to resolve a setuptools incompatibility with the usage of 2to3 in
decorator < 4.0.0. Version 4.2.1 of decorator is currently the
packaged version included in centos 8/stream.

Change-Id: Ib74fd9ebbea7496fd3e7f453e618b218f8859f6f

Author: jharbott

lower-constraints.txt

@@ -7,7 +7,7 @@ cmd2==0.8.0
 coverage==4.0
 cryptography==3.1
 debtcollector==1.2.0
-decorator==3.4.0
+decorator==4.2.1
 deprecation==1.0
 dogpile.cache==0.6.2
 dulwich==0.15.0