[docs] [issue21479] Document TarFile.open() as a classmethod

[Roundup Robot]

Roundup Robot added the comment:

New changeset 37d2a6bcf7ae by Raymond Hettinger in branch '3.4':
Issue 21479: Fix markup for the TarFile.open() classmethod.
http://hg.python.org/cpython/rev/37d2a6bcf7ae

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21479>
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