[misc] Replace flake and pytest with tox in github action

.github/workflows/backend.yml

@@ -18,14 +18,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
+        pip install tox
-        pip install pytest flake8
+    - name: Run validation with tox
-    - name: Lint with flake8
       run: |
-        # stop the build if there are Python syntax errors or undefined names
+        tox
-        flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
-        # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
-        flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
-    - name: Test with pytest
-      run: |
-        pytest