tests: use Python 3.10 in Integration with Regular Sessions presubmit (#1530)

In googleapis/python-spanner#1529, Presubmit
`Kokoro - Test: Integration with Regular Sessions` failed because Python
3.9 is no longer supported in the Python Kokoro CI. This PR fixes the
presubmit by using 3.10 instead.

Author: parthea

packages/google-cloud-spanner/.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"
-}
+}