[Checkins] [zopefoundation/zope.interface] 61c6d5: Make the singleton _empty immutable.

[Jason Madden]

  Branch: refs/heads/issue158
  Home:   https://github.com/zopefoundation/zope.interface
  Commit: 61c6d54ebad0cce407169a27f7a199fc80fdc03a
      https://github.com/zopefoundation/zope.interface/commit/61c6d54ebad0cce407169a27f7a199fc80fdc03a
  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