update default version in noxfile

Author: ohmayr

noxfile.py

@@ -32,7 +32,7 @@
 ISORT_VERSION = "isort==5.11.0"
 LINT_PATHS = ["docs", "google_auth_httplib2.py", "tests", "noxfile.py", "setup.py"]
 
-DEFAULT_PYTHON_VERSION = "3.8"
+DEFAULT_PYTHON_VERSION = "3.9"
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
     "3.9",