[issue30463] Add __slots__ to ABC convenience class

[Serhiy Storchaka]

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


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components: +Library (Lib)
resolution:  -> fixed
stage:  -> resolved
status: open -> closed
type:  -> enhancement
versions: +Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30463>
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