tweaks kokoro to avoid 3.7 and 3.8 NOTE: they are covered by github actions

Author: chalmerlowe

.kokoro/presubmit/presubmit.cfg

@@ -1 +1,6 @@
-# Format: //devtools/kokoro/config/proto/build.proto
+# Format: //devtools/kokoro/config/proto/build.proto
+
+env_vars: {
+    key: "NOX_SESSION"
+    value: "unit-3.9 unit-3.10 unit-3.11 unit-3.12 unit-3.13 unit-3.14 system-3.12 cover lint lint_setup_py blacken mypy docs docfx format"
+}