Try installing python via Travis homebrew addon

.travis.yml

@@ -7,6 +7,11 @@ jobs:
     - language: objective-c
       os: osx
       osx_image: xcode9.2
+      addons:
+        homebrew:
+          packages:
+          - python
+          update: true
     - language: minimal
       dist: xenial
       os: linux

travis/macos-install.sh

@@ -2,9 +2,6 @@
 
 set -ex
 
-export HOMEBREW_NO_INSTALL_CLEANUP=1
-brew update
-brew upgrade python
 which python3
 python3 --version
 which pip3