chore: add constraints file check for python samples (#1288)

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](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

Author: yoshi-automation

.github/header-checker-lint.yml

@@ -1,6 +1,6 @@
 {"allowedCopyrightHolders": ["Google LLC"],
  "allowedLicenses": ["Apache-2.0", "MIT", "BSD-3"],
- "ignoreFiles": ["**/requirements.txt", "**/requirements-test.txt"],
+ "ignoreFiles": ["**/requirements.txt", "**/requirements-test.txt", "**/__init__.py", "samples/**/constraints.txt", "samples/**/constraints-test.txt"],
  "sourceFileExtensions": [
  	"ts", 
  	"js", 

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/google-api-python-client.git",
-        "sha": "9a75e3e99d3f718f08fe197e77a605efc1ad5bb6"
+        "sha": "bd2c5f9c5c5181e21cf970eb6f966afcf0323723"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "705743e66f5c0b24a95f7f30619c9d3ef747b317"
+        "sha": "0a071b3460344886297a304253bf924aa68ddb7e"
       }
     }
   ],