meta: add GitHub action to black --check .

Author: math2001

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Install black
+      run: pip install black
+    - name: Check formating
+      run: black --check .
+