[Checkins] [zopefoundation/zope.interface] be8275: Merge pull request #164 from zopefoundation/issue162

[Jason Madden]

  Branch: refs/heads/lookup-performance
  Home:   https://github.com/zopefoundation/zope.interface
  Commit: be8275645900d3d598d59ef9644983a77ccc8f70
      https://github.com/zopefoundation/zope.interface/commit/be8275645900d3d598d59ef9644983a77ccc8f70
  Author: Jason Madden <jamadden at gmail.com>
  Date:   2020-02-04 (Tue, 04 Feb 2020)

  Changed paths:
    M CHANGES.rst
    M src/zope/interface/declarations.py
    M src/zope/interface/interface.py
    M src/zope/interface/tests/test_adapter.py

  Log Message:
  -----------
  Merge pull request #164 from zopefoundation/issue162

The _empty singleton has no-op subscribe/unsubscribe methods.