buster is 3.6 now (#4003)

diff --git a/Jenkinsfile b/Jenkinsfile
index 0719907..2cd0005 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -46,7 +46,7 @@
     [
         label: 'docker',
         imageName: 'pyca/cryptography-runner-buster',
-        toxenvs: ['py27', 'py35'],
+        toxenvs: ['py27', 'py36'],
     ],
     [
         label: 'docker',