[issue21146] update gzip usage examples in docs

[INADA Naoki]

INADA Naoki added the comment:

Maybe, shutil.copyfileobj() is good.

import gzip
import shutil

with open(src, 'rb') as f_in:
    with gzip.open(dst, 'wb') as f_out:
        shutil.copyfileobj(f_in, f_out)

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nosy: +naoki

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<http://bugs.python.org/issue21146>
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