Use Travis build stages to deploy once to PyPI

I was getting an error with my deploys, because Travis would run the deploy
step once for Python 3.5 and once for Python 2.7 - which meant it was a race,
and whichever one was last would throw an error.

Use Travis build stages to avoid this problem:
https://docs.travis-ci.com/user/build-stages/matrix-expansion/

.travis.yml

@@ -7,11 +7,17 @@ python:
 script:
 - python setup.py test
 
-deploy:
-  provider: pypi
-  user: simonw
-  distributions: bdist_wheel
-  password:
-    secure: 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
-  on:
-    tags: true
+jobs:
+  include:
+    - stage: pypi release
+      python: 3.6
+      script: echo "Deploying to PyPI..."
+      deploy:
+        - provider: pypi
+          user: simonw
+          distributions: bdist_wheel
+          password:
+            secure: Rx6Wl+a2UL+Nr2QQsbtQtrfsZQoC4LmXpDtWyOxFs76aPyC2JmJOb7wYxkmG703LxzZNuUuU9+4WxuA5+/JbyLwT1oQFUBtRfpKIroFa1NKKqQss1RbR+feLufcLT5dJNSkQb+xiKmep5pZnVbRNbmC4z0SFO4/5LJfegSi0pXBbV4IclE7kXSMZS4bYFAb6n9YGfFvwj0o7/GYD6P9MaNwuMqtVsYWZK/S0Bt4wthNc29av8R/jI3++MC3bXs9KC18q5s3x761rWQe/P/u2J++wufqWw9TyrtSVkvQMsaweBuo+0iY4Uiv8m5rPI5xT5rvLC8jdbLnj1FY5xphohEl3/H5vvmW7zBhBFvCSFSirXBkB7fZyX6uoVfe81vNzH/GGNwEi/3E4SVuUk/v2Fm4LBIXfg23O0Gl8aUZnhvcvL5eImadCRCy+dOhLkX9QO7iS6Bu+jjzu5yfqqoIQC96uB9HDn8OZLZ/9W4PlISc1+ARyoCEqeh33mB5IQN41XEbSLbW2STvPS0HYt8mffDDgv/YXjdxUjceGqsUYmksk/6feq2oOdBgwYG1HhOA9dlG7xQsWPCXsaDIZ2olnXnz8AIsZN7jtJiJoUWUF+ZlwLL8zA/wNqLoyohgsKa0Ne4BEpa42XVmhkoWMbl5sJfQzmjS/pQNc9hkv0n1UKYY=
+          on:
+            branch: master
+            tags: true