chore: exclude 'CODEOWNERS' from templated files (#986)

See: googleapis/synthtool#1201

Author: tseaver

owlbot.py

@@ -110,6 +110,7 @@
         "noxfile.py",
         "docs/multiprocessing.rst",
         ".coveragerc",
+        ".github/CODEOWNERS",
         # Include custom SNIPPETS_TESTS job for performance.
         # https://github.com/googleapis/python-bigquery/issues/191
         ".kokoro/presubmit/presubmit.cfg",