test(postfix): env for testing on oldest deps (#6230)

Fixes #6124.

.travis.yml

@@ -34,7 +34,7 @@ matrix:
     - python: "3.5"
       env: TOXENV=mypy
     - python: "2.7"
-      env: TOXENV='py27-{acme,apache,certbot,dns,nginx}-oldest'
+      env: TOXENV='py27-{acme,apache,certbot,dns,nginx,postfix}-oldest'
       sudo: required
       services: docker
     - python: "3.4"

certbot-postfix/certbot_postfix/tests/installer_test.py

@@ -253,7 +253,7 @@ class InstallerTest(certbot_test_util.ConfigTestCase):
                 fake_set.reset_mock()
                 installer.deploy_cert("example.com", "cert_path", "key_path",
                                       "chain_path", "fullchain_path")
-                fake_set.assert_not_called()
+                self.assertFalse(fake_set.called)
 
     @certbot_test_util.patch_get_utility()
     def test_deploy_already_secure(self, mock_util):

tox.ini

@@ -108,6 +108,12 @@ commands =
 setenv =
     {[testenv:py27-oldest]setenv}
 
+[testenv:py27-postfix-oldest]
+commands =
+    {[base]install_and_test} certbot-postfix
+setenv =
+    {[testenv:py27-oldest]setenv}
+
 [testenv:py27_install]
 basepython = python2.7
 commands =