Fix failures from pre-commit run --all, and add to github checks

.github/workflows/main.yml

@@ -55,6 +55,12 @@ jobs:
           python --version
           echo "=====  PANDOC  ====="
           pandoc --version | head -2
+      # see https://github.com/pre-commit/action/#using-this-action
+      - name: pre-commit checks
+        uses: pre-commit/action@v2.0.0
+        env:
+          # it's okay for github to commit to main/master
+          SKIP: no-commit-to-branch
       - name: Run tests
         run: tox -e py${{ matrix.config.python }}