Add python 3.8-dev to travis and tox (#15347)

* Add Python 3.8-dev tox tests.

* Allow failures on 3.8-dev

* Allow failures on 3.8-dev take2

* Only run on pushes to dev

.travis.yml

@@ -19,9 +19,14 @@ matrix:
     - python: "3.7"
       env: TOXENV=py37
       dist: xenial
-  # allow_failures:
-  #   - python: "3.5"
-  #     env: TOXENV=typing
+    - python: "3.8-dev"
+      env: TOXENV=py38
+      dist: xenial
+      if: branch = dev AND type = push
+  allow_failures:
+    - python: "3.8-dev"
+      env: TOXENV=py38
+      dist: xenial
 
 cache:
   directories:

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35, py36, py37, lint, pylint, typing
+envlist = py35, py36, py37, py38, lint, pylint, typing
 skip_missing_interpreters = True
 
 [testenv]