Split the oldbs4 into a separate Travis job

.travis.yml

@@ -4,6 +4,13 @@ python:
   - "3.6"
   - "3.7"
   - "3.8"
+jobs:
+  include:
+    - python: "3.5"
+      env: TOXENV=py35-oldbs4
+  allow_failures:
+    - python: "3.5"
+      env: TOXENV=py35-oldbs4
 # command to install dependencies
 addons:
   apt:

tox.ini

@@ -31,3 +31,8 @@ deps =
     dnspython
     future
     {[base]deps}
+
+[travis]
+# don't run py35-oldbs4 by default in the Python 3.5 job
+python =
+  3.5: py35