Update Standard Version.

I read through the changelogs here and as far as I can tell we're as
compliant with the changes as we were with the previous standards.

Please open issues for Standards violations. They won't always be
addressed but we can note them in the README.

stdeb/util.py

@@ -1620,7 +1620,7 @@ def build_dsc(debinfo,
 %(uploaders)sSection: %(debian_section)s
 Priority: optional
 Build-Depends: %(build_depends)s
-Standards-Version: 3.9.1
+Standards-Version: 4.7.0
 %(source_stanza_extras)s
 
 %(control_py2_stanza)s