Add GitHub Action to lint Python code

https://github.com/mingrammer/diagrams/actions
https://github.com/cclauss/diagrams/actions

.github/workflows/lint_python.yml

@@ -0,0 +1,18 @@
+name: lint_python
+on:
+  pull_request:
+  push:
+  #  branches: [master]
+jobs:
+  lint_python:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - uses: actions/setup-python@master
+      - run: pip install black codespell flake8 isort pytest
+      - run: black . --diff || true
+      - run: codespell --quiet-level=2 || true  # --ignore-words-list="" --skip=""
+      - run: flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+      - run: isort --recursive . || true
+      - run: pip install -r requirements.txt || true
+      - run: pytest . || true