[Checkins] [zopefoundation/Products.ZCatalog] bf9782: Fix 100 (#103)

[Eric Wohnlich]

  Branch: refs/heads/master
  Home:   https://github.com/zopefoundation/Products.ZCatalog
  Commit: bf9782883bf143748d4b08fb42d11502fd7b21f6
      https://github.com/zopefoundation/Products.ZCatalog/commit/bf9782883bf143748d4b08fb42d11502fd7b21f6
  Author: Eric Wohnlich <eric.wohnlich at gmail.com>
  Date:   2020-05-08 (Fri, 08 May 2020)

  Changed paths:
    M CHANGES.rst
    M src/Products/PluginIndexes/FieldIndex/tests.py
    M src/Products/PluginIndexes/unindex.py

  Log Message:
  -----------
  Fix 100 (#103)

* proposed fix for #100, case where index value is changed to None after previously being indexed

* remove pycharm junk

* remove old comment as no longer relevant in BTrees. But we still want
to not index None values

* use assertRaises instead of try/except for expected KeyError

Co-authored-by: wohnlice <wohnlice at imsweb.com>