Update GitHub Actions

* Run on all pushes, but only publish on master branch
* Check style

.github/workflows/publish.yml

@@ -1,8 +1,6 @@
 name: Publish
 on:
   push:
-    branches:
-      - master
   workflow_dispatch:
 jobs:
   build:
@@ -16,6 +14,9 @@ jobs:
       run: |
         python -m pip install poetry
         poetry install --no-dev
+    - name: Check style
+      run: |
+        poetry run black . --check
     - name: Generate
       run: |
         poetry run python festivus.py
@@ -30,3 +31,4 @@ jobs:
         git add -f .
         git commit -m 'Deploy'
         git push --force origin HEAD
+      if: github.ref == 'refs/heads/master' && github.event_name == 'push'