[gitlab-ci] add py3-from-forge pipeline

.gitlab-ci.yml

@@ -9,6 +9,9 @@ flake8:
 py3:
   script: tox -e py3
 
+py3-from-forge:
+  script: tox -e py3-from-forge
+
 check-manifest:
   script: tox -e check-manifest