revert manual update of noxfile

Author: ohmayr

noxfile.py

@@ -32,7 +32,7 @@
 ISORT_VERSION = "isort==5.11.0"
 LINT_PATHS = ["docs", "google", "tests", "noxfile.py", "setup.py"]
 
-DEFAULT_PYTHON_VERSION = "3.9"
+DEFAULT_PYTHON_VERSION = "3.8"
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
     "3.7",
@@ -75,11 +75,7 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit-3.9",
-    "unit-3.10",
-    "unit-3.11",
-    "unit-3.12",
-    "unit-3.13",
+    "unit",
     "system",
     "mypy",
     "cover",