Introduce lint pipeline and fix some minor issues flake8 complained about

.github/workflows/ci.yml

@@ -0,0 +1,16 @@
+name: ci
+
+on: [ push, pull_request ]
+
+jobs:
+ lint:
+  runs-on: ubuntu-latest
+  steps:
+  - name: Checkout
+    uses: actions/checkout@v2
+  - name: Set up Python 3.8
+    uses: actions/setup-python@v2
+    with:
+     python-version: 3.8
+  - run: pip install flake8
+  - run: flake8 run.py core