fedora comes with a py36 now (#3781)

diff --git a/Jenkinsfile b/Jenkinsfile
index d953a48..d1ea0ba 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -78,7 +78,7 @@
     [
         label: 'docker',
         imageName: 'pyca/cryptography-runner-fedora',
-        toxenvs: ['py27', 'py35'],
+        toxenvs: ['py27', 'py36'],
     ],
     [
         label: 'docker',