Add GitHub Actions for black

This GitHub Action checks that code in pull request
is formatted with black.

See https://black.readthedocs.io/en/stable/github_actions.html

Author: PokhodenkoSA

.github/workflows/black.yml

@@ -0,0 +1,28 @@
+# This is a workflow to format Python code with black formatter
+
+name: black
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "black"
+  black:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+      # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
+      - uses: actions/checkout@v2
+      # Set up a Python environment for use in actions
+      - uses: actions/setup-python@v2
+
+      # Run black code formatter
+      - uses: psf/black@stable