Add back Six for internal_ci scripts (#31390)

tools/internal_ci/helper_scripts/prepare_build_linux_rc

@@ -41,6 +41,8 @@ export DOCKERHUB_ORGANIZATION=grpctesting
 
 git submodule update --init
 
+python3 -m pip install six
+
 # check whether /tmpfs is mounted correctly
 (mount | grep -q 'on /tmpfs ') || (mount; echo 'BAD KOKORO WORKER WARNING: it seems that /tmpfs volume with scratch disk is not mounted in the kokoro worker. This can result in unexpected "out of disk space" errors.')
 

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -48,7 +48,7 @@ export HOMEBREW_NO_AUTO_UPDATE=1
 brew config
 
 # Add GCP credentials for BQ access
-pip install --user google-api-python-client oauth2client
+pip install --user google-api-python-client oauth2client six==1.16.0
 export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/GrpcTesting-d0eeee2db331.json
 DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
 

tools/internal_ci/helper_scripts/prepare_build_windows.bat

@@ -85,7 +85,7 @@ If "%PREPARE_BUILD_INSTALL_DEPS_PYTHON%" == "true" (
 )
 
 @rem Needed for uploading test results to bigquery
-python -m pip install google-api-python-client oauth2client || goto :error
+python -m pip install google-api-python-client oauth2client six==1.16.0 || goto :error
 
 git submodule update --init || goto :error