Updates owlbot, removing reference to 3.8

Author: chalmerlowe

owlbot.py

@@ -28,7 +28,7 @@
 # Add templated files
 # ----------------------------------------------------------------------------
 templated_files = common.py_library(
-    system_test_python_versions=["3.8"],
+    system_test_python_versions=["3.9"],
     cov_level=100,
     intersphinx_dependencies={
         "pandas": "https://pandas.pydata.org/pandas-docs/stable/"