Move Black from Travis to Github Actions (#390)

.github/workflows/black.yaml

@@ -0,0 +1,21 @@
+name: Black
+
+on: ["push", "pull_request"]
+
+jobs:
+  black:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: '3.x'
+          architecture: 'x64'
+
+      - name: Checkout
+        uses: actions/checkout@v1
+
+      - name: Black Code Formatter
+        run: |
+          pip install black
+          black --diff --check msgpack/ test/