[issue20222] unittest.mock-examples doc uses builtin file which is removed in Python 3

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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assignee: docs at python -> terry.reedy
versions:  -Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20222>
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