Debian sid is python3.6 now (#3968)

* Debian sid is python3.6 now

* Workaround because apparently measuring coverage correctly isn't a legitimate use case
diff --git a/Jenkinsfile b/Jenkinsfile
index 0e4c60c..60195a6 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -51,7 +51,7 @@
     [
         label: 'docker',
         imageName: 'pyca/cryptography-runner-sid',
-        toxenvs: ['py27', 'py35'],
+        toxenvs: ['py27', 'py36'],
     ],
     [
         label: 'docker',