CI: add lints

.github/workflows/lint.yml

@@ -0,0 +1,8 @@
+name: Lint
+on: [push, pull_request]
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: psf/black@stable

.github/workflows/test.yml

@@ -1,5 +1,5 @@
-name: build docker image and run tests inside
-on: push
+name: Tests
+on: [push, pull_request]
 jobs:
   tests:
     runs-on: ubuntu-latest