[issue23726] Don't enable GC for classes that don't add new fields

[Serhiy Storchaka]

Changes by Serhiy Storchaka <storchaka at gmail.com>:


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components: +Interpreter Core
nosy: +serhiy.storchaka
stage:  -> patch review
versions: +Python 3.5 -Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23726>
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