[issue11333] Add empty __slots__ to collections.abc abstract base classes

[Giampaolo Rodola']

Changes by Giampaolo Rodola' <g.rodola at gmail.com>:


----------
nosy: +giampaolo.rodola

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11333>
_______________________________________