chore: Add header checker config to python library synth (#910)

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 :)

Author: leahecole

synthtool/gcp/templates/python_library/.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"
+ ]
+}