[issue9139] Add examples for str.format()

[Eric Smith]

Eric Smith <eric at trueblade.com> added the comment:

I think I'd just delete both instances of "now". And now that I read it more critically, "position" should be "positional". How's this rewording:

.. versionchanged:: 2.7
   The positional argument specifiers can be omitted, so ``'{} {}'`` is
   equivalent to ``'{0} {1}'``.

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<http://bugs.python.org/issue9139>
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