Add flake8 in the workflow

.github/workflows/flake8.yml

@@ -0,0 +1,26 @@
+name: Flake8 check.
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  check:
+    name: Flake8 check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      # Flake8 check
+      - name: Install flake8
+        run: pip install flake8
+      - name: Run flake8
+        uses: suo/flake8-github-action@releases/v1
+        with:
+          # NOTE: this needs to be the same as the job name
+          checkName: 'Flake8 check'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+