Only run flake8 annotation if there are *.py files in diff

.github/workflows/pr-annotate-with-flake8.yml

@@ -11,19 +11,43 @@ on:
     branches: [ develop ]
 
 jobs:
-  build:
-
+  check_py:
     runs-on: ubuntu-18.04
-
+    
     steps:
-    - uses: actions/checkout@v2
+    - name: Checkout
+      uses: actions/checkout@v2
       with:
         fetch-depth: 0
-    - name: Set up Python 3.7
-      uses: actions/setup-python@v2
-      with:
-        python-version: 3.7
-    - name: Annotate with Flake8
-      uses: "tayfun/flake8-your-pr@master"
+
+    - name: Check for PY files
       env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        BASE_REF: ${{github.base_ref}}
+      run: |
+        # Will exit with non-zero if no filenames ending in ".py" are in
+        # the diff.
+        git diff --name-only "refs/remotes/origin/${BASE_REF}" -- | egrep -v '.py$'
+
+  flake8_annotate:
+    - name: Annotate PR with Flake8
+      needs: check_py
+      # Only run if the check_py succeeded
+      if: ${{ success() }}
+
+      runs-on: ubuntu-18.04
+
+      steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Set up Python 3.7
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.7
+
+      - name: Annotate with Flake8
+        uses: "tayfun/flake8-your-pr@master"
+        needs:
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}