chore: add constraints file check for python samples

[leahecole]

This is the sibling PR to GoogleCloudPlatform/python-docs-samples#5611
and this is the issue opened for it GoogleCloudPlatform/python-docs-samples#5549

If you look at the files in this example repo, you'll see that renovate successfully opened a PR on three constraints files in samples directories and subdirectories, and properly ignored constraints files at the root level

cc @tswast

TODO:

• update renovate to check for samples/constraints.txt dependency updates
• run lint locally to double check that I'm not introducing lint error

[leahecole]

I converted this to a draft because we're talking in the samples owners group about if I also need to add an update to the renovate template, and if so, what that will look like. I'll make it ready to review once that's been decided.

[tswast]

LGTM