Travis: temporarily disable Python 3.8

There seems to be a compatibility error in Werkzeug on the prerelease of
3.8.

Author: sampsyo

.travis.yml

@@ -24,9 +24,9 @@ matrix:
         - python: 3.7
           env: {TOX_ENV: py37-test}
           dist: xenial
-        - python: 3.8-dev
-          env: {TOX_ENV: py38-test}
-          dist: xenial
+        # - python: 3.8-dev
+        #   env: {TOX_ENV: py38-test}
+        #   dist: xenial
         # - python: pypy
         #   - env: {TOX_ENV: pypy-test}
         - python: 3.4