Bump Standards-Version to 4.2.1.

debian/control

@@ -10,7 +10,7 @@ Build-Depends: debhelper (>= 9),
  python-sphinx (>= 1.0.7+dfsg),
  python3-all (>= 3.1.2),
  python3-setuptools
-Standards-Version: 3.9.3
+Standards-Version: 4.2.1
 Homepage: https://github.com/sunlightlabs/validictory
 Vcs-Git: https://salsa.debian.org/python-team/modules/python-validictory.git
 Vcs-Browser: https://salsa.debian.org/python-team/modules/python-validictory