Python write_stream sample returns AttributeError when run

Hi,

I am running a local timebase server using docker using the following command:
```
docker run --rm -d -p 8011:8011 --name=timebase-server --ulimit nofile=65536:65536 finos/timebase-ce-server:6.1
```

This allows me to run the `list_streams.py` sample successfully.

However, when I run the `write_stream.py` sample I get the following error:
```
Connected to dxtick://localhost:8011
Connection dxtick://localhost:8011 closed.
Traceback (most recent call last):
File "write_stream.py", line 28, in
loader = stream.createLoader(tbapi.LoadingOptions())
AttributeError: 'NoneType' object has no attribute 'createLoader'
```

A cursory look at the code leads me to believe that the `bitfinex` stream does not exist in the timebase server so the person who wrote this sample was probably pointing to a timebase server that had already been primed in some way. So this sample does not stand by itself and needs some unknown setup.

I have attempted to create a stream instead of opening it but I get further errors due to the stream not being setup correctly (as expected):
```
No message types defined for stream at DxApiImpl::TickLoaderImpl *DxApiImpl::TickLoaderImpl::open(
```

I hope that is enough info to reproduce.

Reproduza a falha executando write_stream.py contra o servidor iniciado com o comando Docker documentado e, em seguida, compare-o com list_streams.py. Inspecione a busca do stream em write_stream.py e verifique a configuração esperada do stream do exemplo. Considera-se concluído quando o exemplo funciona contra um servidor recém-iniciado ou documenta claramente a configuração necessária sem o AttributeError.

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