Potential resource contention with unclosed streams

[parrottsquawk]

https://github.com/amazon-ion/ion-python/blame/7e0409c49ce2bfb023ecab384286a5581d185756/src-python/amazon/ion/writer_text.py#L224

Please consider closing out the stream after the content has been retrieved.

[jobarr-amzn]

Thanks for the issue. I've tossed out a PR to ensure immediate closure of the BytesIO object and release of its buffer, is that what you meant?

I'm not very familiar with ion-python so bear with me :)