[docs] [issue21146] update gzip usage examples in docs

[Wolfgang Maier]

Wolfgang Maier added the comment:

well, buffering is not the issue here. It's that the file iterator used in the current example is line-based, so whatever the buffer size you're doing unnecessary inspection to find and split on line terminators.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21146>
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