tests: use Python 3.10 in Integration with Regular Sessions presubmit

Author: parthea

.kokoro/presubmit/integration-regular-sessions-enabled.cfg

@@ -3,7 +3,7 @@
 # Only run a subset of all nox sessions
 env_vars: {
     key: "NOX_SESSION"
-    value: "unit-3.9 unit-3.14 system-3.14"
+    value: "unit-3.10 unit-3.14 system-3.14"
 }
 
 env_vars: {
@@ -19,4 +19,4 @@ env_vars: {
 env_vars: {
     key: "GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW"
     value: "false"
-}
+}