Allow failures on dev branch of Python 3.8

Since Python 3.8 is going through significant changes, we should
allow failures and not block other commits from merging because
of 3.8 compatibility.

Signed-off-by: Eric Brown <browne@vmware.com>

.travis.yml

@@ -31,6 +31,11 @@ matrix:
       sudo: true
     - python: pypy
       env: TOXENV=pypy
+  allow_failures:
+    - python: 3.8-dev
+      env: TOXENV=py38
+      dist: xenial
+      sudo: true
 
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