Keras CI fix (#5486)

* Keras CI fix

* pre-commit fixes

* Update ci-testing.yml

Co-authored-by: pre-commit <pre-commit@example.com>

.github/workflows/ci-testing.yml

           restore-keys: |
             ${{ runner.os }}-${{ matrix.python-version }}-pip-
 
+      # Known Keras 2.7.0 issue: https://github.com/ultralytics/yolov5/pull/5486
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
           pip install -qr requirements.txt -f https://download.pytorch.org/whl/cpu/torch_stable.html
-          pip install -q onnx tensorflow-cpu  # for export
+          pip install -q onnx tensorflow-cpu keras==2.6.0  # for export
           python --version
           pip --version
           pip list

tutorial.ipynb

       "outputs": []
     }
   ]
-}
+}