[.travis.yml] eliminate duplicate Travis builds on PR that originate from in repo branches

Author: chrissimpkins

.travis.yml

@@ -39,5 +39,16 @@ matrix:
       env: TOX_ENV=py37
       script: tox -e $TOX_ENV
 
+# The following prevents Travis from running CI on pull requests that come from a
+# branch in the same repository. Without this, it will run the same CI for the
+# pull request branch _and_ the pull request itself, which makes no sense.
+branches:
+  only:
+    - master
+    # We want to build wip/* branches since these are not usually used for PRs
+    - /^wip\/.*$/
+    # We want to build version tags as well.
+    - /^v\d+\.\d+.*$/
+
 notifications:
   email: false