[issue13424] Add examples for open’s new opener argument

[Ezio Melotti]

Ezio Melotti added the comment:

There's also a Sphinx warning that should be fixed:
3.3/Doc/library/functions.rst:955: WARNING: undefined label: dir_fd (if the link has no caption the label must precede a section header)

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nosy: +ezio.melotti

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13424>
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