message: [travis-ci] fix install python
  Unlink python2 before brew install, which fails for unable to link
  python as python3.
diff: @@ -56,6 +56,7 @@ case $BUILD_TARGET in
  macOS)
  # On Travis, brew install fails when a package is already installed, so use reinstall here instead of ./script/bootstrap
  + brew unlink python@2 || true
  brew reinstall boost cmake cpputest dbus jsoncpp ninja
  OTBR_OPTIONS='-DOTBR_MDNS=OFF' ./script/test build
  ;;
  