fix: fail nox session if python version is missing (#6764)
fix: fail nox session if python version is missing (#6764)

* fix: fail nox session if python version is missing

* chore: modify spacing
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples