Make lint jobs only run on merge requests

.gitlab-ci.yml

@@ -25,6 +25,8 @@ black:
   stage: lint
   script:
     - black --check --diff .
+  only:
+    - merge_requests
 
 flake8:
   tags:
@@ -32,6 +34,8 @@ flake8:
   stage: lint
   script:
     - flake8 .
+  only:
+    - merge_requests
 
 pylint:
   tags:
@@ -39,3 +43,5 @@ pylint:
   stage: lint
   script:
     - pylint app *.py
+  only:
+    - merge_requests