Python: noxfile support for constraints #839

In the BigQuery libraries, we have more dependencies in the library than some other libraries (mostly optional, via "extras"). I'd like to be able to have some tests the depend on specific versions so that we can ensure the version ranges in setup.py are correct.

In google-cloud-bigquery we do this by adding testing/constraints-X.Y.txt files, where X.Y==session.python in nox.

It'd be nice if the template had support for this feature so that we could use constraints.txt files in the other BigQuery libraries without too much manual regex.