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

[Terry J. Reedy]

Terry J. Reedy added the comment:

I think /file/open/ would work.

----------
nosy: +michael.foord, terry.reedy

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