chore(internal/kokoro): fix gcp-docuploader install
@@ -19,7 +19,9 @@ set -eo pipefail
# Display commands being run.
set -x
-python3 -m pip install "gcp-docuploader<2019.0.0"
+python3 -m pip install --upgrade pip
+# Workaround for six 1.15 incompatibility issue.
+python3 -m pip install --use-feature=2020-resolver "gcp-docuploader<2019.0.0"
cd github/google-cloud-go/internal/godocfx
go install
