[issue18575] Fixing tarfile._mode when using gzip via ":gz"

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka at gmail.com>:


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dependencies: +Multivolume support in tarfile module
nosy: +serhiy.storchaka
stage:  -> patch review
type:  -> behavior

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18575>
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