[Checkins] [zopefoundation/zope.interface] cf19f8: Make the singleton _empty immutable.

[Jason Madden]

  Branch: refs/heads/issue158
  Home:   https://github.com/zopefoundation/zope.interface
  Commit: cf19f8be529670929e4e2a6f4722b41a8a543e51
      https://github.com/zopefoundation/zope.interface/commit/cf19f8be529670929e4e2a6f4722b41a8a543e51
  Author: Jason Madden <jamadden at gmail.com>
  Date:   2020-01-27 (Mon, 27 Jan 2020)

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

  Log Message:
  -----------
  Make the singleton _empty immutable.

Fixes #158