update python version (#6399)

Author: awaelchli

notebooks/06-mnist-tpu-training.ipynb

@@ -80,7 +80,7 @@
         "id": "AYGWh10lRaF1"
       },
       "source": [
-        "! pip install cloud-tpu-client==0.10 https://storage.googleapis.com/tpu-pytorch/wheels/torch_xla-1.7-cp36-cp36m-linux_x86_64.whl"
+        "! pip install cloud-tpu-client==0.10 https://storage.googleapis.com/tpu-pytorch/wheels/torch_xla-1.7-cp37-cp37m-linux_x86_64.whl"
       ],
       "execution_count": null,
       "outputs": []