travis: use __token__ user instead of @token

pypi API token is currently beta feature, since
pypi/warehouse#6342 pypi "token user" has changed to
"__token__".

.travis.yml

@@ -25,7 +25,7 @@ jobs:
       if: repo == "philpep/testinfra" AND tag IS present
       deploy:
         provider: pypi
-        user: "@token"
+        user: "__token__"
         on:
           tags: true
         distributions: "sdist bdist_wheel"