Run tests on buster (#3735)

diff --git a/Jenkinsfile b/Jenkinsfile
index c7710d6..8c3c55f 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -40,12 +40,17 @@
     ],
     [
         label: 'docker',
-        imageName: 'pyca/cryptography-runner-sid',
+        imageName: 'pyca/cryptography-runner-stretch',
         toxenvs: ['py27', 'py35'],
     ],
     [
         label: 'docker',
-        imageName: 'pyca/cryptography-runner-stretch',
+        imageName: 'pyca/cryptography-runner-buster',
+        toxenvs: ['py27', 'py35'],
+    ],
+    [
+        label: 'docker',
+        imageName: 'pyca/cryptography-runner-sid',
         toxenvs: ['py27', 'py35'],
     ],
     [