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", 

renovate.json

@@ -2,5 +2,8 @@
   "extends": [
     "config:base",  ":preserveSemverRanges"
   ],
-  "ignorePaths": [".pre-commit-config.yaml"]
+  "ignorePaths": [".pre-commit-config.yaml"],
+  "pip_requirements": {
+    "fileMatch": ["requirements-test.txt", "samples/[\\S/]*constraints.txt", "samples/[\\S/]*constraints-test.txt"]
+  }
 }

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-bigquery-sqlalchemy.git",
-        "sha": "101153a14706ae325893f3ddf6dbd934c4ef65a0"
+        "sha": "29c7fb09b345a309bdbc7b6052e4f3f5028e2498"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "5b5bf6d519b2d658d9f2e483d9f6f3d0ba8ee6bc"
+        "sha": "0a071b3460344886297a304253bf924aa68ddb7e"
       }
     }
   ],