:green_heart: :snake: :apple: use python3 for osx

Former-commit-id: 242f286b20a2b25c2a1be9267eb1aab7e3ea8e0d

.travis.yml

@@ -47,7 +47,7 @@ matrix:
   - os: osx
     compiler: clang
     env:
-    - MATRIX_EVAL="export CONFIG=Debug && CHECK_UNDEFINED=ON && PYTHON=python"
+    - MATRIX_EVAL="export CONFIG=Debug && CHECK_UNDEFINED=ON && PYTHON=python3"
 
 install:
 - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install ccache; fi