[ci] unlink python2 to fix OSX setup

extra/azure-pipelines/build-mac.yml

@@ -16,6 +16,7 @@ jobs:
       - script: |
           set -ex
           brew update || brew update || brew update
+          brew unlink python@2
           brew bundle --file=tests/Brewfile --no-upgrade
         displayName: Install dependencies
       - template: install-neko-snapshot.yaml