GitHub Action to lint Python code

Because Travis CI is no longer free for open source.

.github/workflows/ruff.yml

@@ -0,0 +1,14 @@
+# https://beta.ruff.rs
+name: ruff
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+jobs:
+  ruff:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - run: pip install --user ruff
+    - run: ruff --format=github --ignore=F401 --target-version=py37 .