chore: add system 3.9 tests as required

.github/sync-repo-settings.yaml

@@ -10,11 +10,5 @@ branchProtectionRules:
     - 'Kokoro'
     - 'cla/google'
     - 'Kokoro system-3.14'
+    - 'Kokoro system-3.9'
     - 'OwlBot Post Processor'
-- pattern: python2
-  requiresCodeOwnerReviews: true
-  requiresStrictStatusChecks: true
-  requiredStatusCheckContexts:
-    - 'Kokoro'
-    - 'cla/google'
-    - 'Kokoro system-2.7'

.kokoro/presubmit/presubmit.cfg

@@ -1,6 +1,7 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
-# Disable system tests.
+# Disable system tests in this presubmit because they are run in separate
+# presubmit jobs, whose configs are in system-3.xx.cfg files.
 env_vars: {
     key: "RUN_SYSTEM_TESTS"
     value: "false"

.kokoro/presubmit/system-3.9.cfg

@@ -4,4 +4,10 @@
 env_vars: {
     key: "NOX_SESSION"
     value: "system-3.9"
+}
+
+# Credentials needed to test universe domain.
+env_vars: {
+  key: "SECRET_MANAGER_KEYS"
+  value: "client-library-test-universe-domain-credential"
 }