* Refactor code using pyupgrade for Python 3.6
This diff is the result of applying the following command to the
project:
```shell
find . -type f -name "*.py" -exec pyupgrade --py36-plus '{}' +
```
* Simplify yield from
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| .. | ||
| test_server.proto | ||
| test_server_pb2.py | ||
| test_server_pb2_grpc.py | ||