chore: add constraints file check for python samples (#87)
chore: add constraints file check for python samples (#87)

This is the sibling PR to https://github.com/GoogleCloudPlatform/python-docs-samples/pull/5611
and this is the issue opened for it https://github.com/GoogleCloudPlatform/python-docs-samples/issues/5549

If you look at the files in [this example repo](https://github.com/leahecole/testrepo-githubapp/pull/31/files), 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:

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

Source-Author: Leah E. Cole <a5082685b0b89abc4f85079ca3a58177707489f8@users.noreply.github.com>
Source-Date: Fri Apr 9 22:50:04 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 0a071b3460344886297a304253bf924aa68ddb7e
Source-Link: https://github.com/googleapis/synthtool/commit/0a071b3460344886297a304253bf924aa68ddb7e
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples