[issue21231] Issue a python 3 warning when old style classes are defined.

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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components: +Interpreter Core
nosy: +ezio.melotti
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21231>
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