[docs] [issue12982] Document that importing .pyo files needs python -O

[Roundup Robot]

Roundup Robot added the comment:

New changeset 8e9dc3e4ea91 by Berker Peksag in branch '3.5':
Issue #12982: Thanks to PEP 488, Python no longer creates .pyo files
https://hg.python.org/cpython/rev/8e9dc3e4ea91

New changeset 1455851e7332 by Berker Peksag in branch 'default':
Issue #12982: Merge from 3.5
https://hg.python.org/cpython/rev/1455851e7332

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nosy: +python-dev

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<http://bugs.python.org/issue12982>
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