chore: add constraints file check for python samples

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

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 <6719667+leahecole@users.noreply.github.com>
Source-Date: Fri Apr 9 22:50:04 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 0a071b3460344886297a304253bf924aa68ddb7e
Source-Link: googleapis/synthtool@0a071b3

.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

@@ -4,21 +4,21 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-audit-log.git",
-        "sha": "f8e535c13a3546ce382cc7e5fdddc42458a97f80"
+        "sha": "e14194538cb4978b97b683c116d577074f485944"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ff39353f34a36e7643b86e97724e4027ab466dc6"
+        "sha": "0a071b3460344886297a304253bf924aa68ddb7e"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ff39353f34a36e7643b86e97724e4027ab466dc6"
+        "sha": "0a071b3460344886297a304253bf924aa68ddb7e"
       }
     }
   ],