[Checkins] [zopefoundation/Missing] 131942: Restore `Missing._Missing` module.

[GitHub]

  Branch: refs/heads/restore__Missing
  Home:   https://github.com/zopefoundation/Missing
  Commit: 131942bc3e4355c5d5a358393ebaa0dc7e9a9049
      https://github.com/zopefoundation/Missing/commit/131942bc3e4355c5d5a358393ebaa0dc7e9a9049
  Author: Michael Howitz <mh at gocept.com>
  Date:   2017-08-24 (Thu, 24 Aug 2017)

  Changed paths:
    M CHANGES.rst
    A src/Missing/_Missing.py

  Log Message:
  -----------
  Restore `Missing._Missing` module.

This is needed for pickle compatibility with the old C extension version.