python: generated noxfile.py refers to nonexistent noxfile_config.py

[hegemonic]

Environment details

n/a

Steps to reproduce

https://github.com/googleapis/synthtool/blob/95d9289a/synthtool/gcp/templates/python_samples/noxfile.py.j2#L35-L40

This template file tells users to copy noxfile_config.py rather than editing noxfile.py. But it appears that the python_samples template doesn't generate a file named noxfile_config.py. For an example, see https://github.com/googleapis/python-iam.

We should clarify these instructions and make sure they're consistent with the generated output.