[Checkins] [zopefoundation/MultiMapping] 018a7f: Add `__contains__` support.

[GitHub]

  Branch: refs/heads/add-contains
  Home:   https://github.com/zopefoundation/MultiMapping
  Commit: 018a7f10935cda750156b4656bd5d48c23d6f3a0
      https://github.com/zopefoundation/MultiMapping/commit/018a7f10935cda750156b4656bd5d48c23d6f3a0
  Author: Hanno Schlichting <hanno at hannosch.eu>
  Date:   2017-05-12 (Fri, 12 May 2017)

  Changed paths:
    M CHANGES.txt
    M src/MultiMapping/__init__.py
    M src/MultiMapping/tests.py

  Log Message:
  -----------
  Add `__contains__` support.

This is only added to the Python version, as it assumes #3 will be
merged beforehand.