feat: switch to Merge Request pipelines as default

templates/gitlab-ci-python.yml

@@ -13,11 +13,11 @@
 # program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
 # Floor, Boston, MA  02110-1301, USA.
 # =========================================================================================
-# default workflow rules
+# default workflow rules: Merge Request pipelines
 workflow:
   rules:
-    # exclude merge requests
-    - if: $CI_MERGE_REQUEST_ID
+    - if: '$CI_MERGE_REQUEST_ID'
+    - if: '$CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS'
       when: never
     - when: always