chore: Add header checker config to python library synth

Now that we have it working in [python-docs-samples](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/header-checker-lint.yml) we should consider adding it to the 🐍 libraries :)

Source-Author: Leah E. Cole <[email protected]>
Source-Date: Mon Jan 25 13:24:08 2021 -0800
Source-Repo: googleapis/synthtool
Source-Sha: 573f7655311b553a937f9123bee17bf78497db95
Source-Link: googleapis/synthtool@573f765

Author: yoshi-automation

.github/header-checker-lint.yml

@@ -0,0 +1,15 @@
+{"allowedCopyrightHolders": ["Google LLC"],
+ "allowedLicenses": ["Apache-2.0", "MIT", "BSD-3"],
+ "ignoreFiles": ["**/requirements.txt", "**/requirements-test.txt"],
+ "sourceFileExtensions": [
+ 	"ts", 
+ 	"js", 
+ 	"java", 
+ 	"sh", 
+ 	"Dockerfile", 
+ 	"yaml", 
+ 	"py",
+ 	"html",
+ 	"txt"
+ ]
+}

synth.metadata

@@ -18,7 +18,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "fb53b6fb373b7c3edf4e55f3e8036bc6d73fa483"
+        "sha": "573f7655311b553a937f9123bee17bf78497db95"
       }
     }
   ],
@@ -28,6 +28,7 @@
     ".github/ISSUE_TEMPLATE/feature_request.md",
     ".github/ISSUE_TEMPLATE/support_request.md",
     ".github/PULL_REQUEST_TEMPLATE.md",
+    ".github/header-checker-lint.yml",
     ".github/release-please.yml",
     ".github/snippet-bot.yml",
     ".kokoro/build.sh",