Add flake8 to review CI to check Python files. (#14582)

* Add flake8 to review CI to check Python files.

We used to have this in our old Travis jobs, but it got removed at some
point before the switch to GitHub Actions and we never re-added it.

* Fix typo.

.flake8

@@ -0,0 +1,2 @@
+[flake8]
+max-line-length=120

.github/workflows/review.yml

@@ -16,6 +16,7 @@ jobs:
     outputs:
       actionlint: ${{ steps.actionlint.outputs.run }}
       eslint: ${{ steps.eslint.outputs.run }}
+      flake8: ${{ steps.flake8.outputs.run }}
       hadolint: ${{ steps.hadolint.outputs.run }}
       shellcheck: ${{ steps.shellcheck.outputs.run }}
       yamllint: ${{ steps.yamllint.outputs.run }}
@@ -47,6 +48,17 @@ jobs:
           else
             echo "run=false" >> "${GITHUB_OUTPUT}"
           fi
+      - name: Check files for flake8
+        id: flake8
+        run: |
+          if [ "${{ contains(github.event.pull_request.labels.*.name, 'run-ci/flake8') }}" = "true" ]; then
+            echo "run=true" >> "${GITHUB_OUTPUT}"
+          elif git diff --name-only origin/${{ github.base_ref }} HEAD | grep -Eq '.*\.py' ; then
+            echo "run=true" >> "${GITHUB_OUTPUT}"
+            echo 'Python files have changed, need to run flake8.'
+          else
+            echo "run=false" >> "${GITHUB_OUTPUT}"
+          fi
       - name: Check files for hadolint
         id: hadolint
         run: |
@@ -118,6 +130,27 @@ jobs:
           reporter: github-pr-check
           eslint_flags: '.'
 
+  flake8:
+    name: flake8
+    needs: prep-review
+    if: needs.prep-review.outputs.flake8 == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Git clone repository
+        uses: actions/checkout@v3
+        with:
+          submodules: recursive
+          fetch-depth: 0
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.10"
+      - name: Run flake8
+        uses: reviewdog/action-flake8@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          reporter: github-pr-check
+
   hadolint:
     name: hadolint
     needs: prep-review