Test only last Python version with PostgreSQL

(grafted from 23d775fc0b5ad1e5082993f48bba3bfb5dc554d9)

--HG--
branch : 6.0

.gitlab-ci.yml

@@ -181,6 +181,9 @@ check-ci-modules:
     - name: ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/postgres
       alias: postgresql
       command: ["-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
+  parallel:
+    matrix:
+      - PYTHON_VERSION: ["3.9"]
 
 .test-postgis:
   extends: .test-db