[CI] #97: Trigger PyPi release job only on tag

- Prevents the creation of two jobs: one from the tag and the other from the commit
- Might fix non-working PyPi releases not made from the job with the tag

.gitlab-ci.yml

@@ -99,4 +99,5 @@ releases:
   <<: *push_to_github
   stage: release
   when: manual
-
+  only:
+    - tags