[Checkins] [zopefoundation/transaction] 6e9761: Make setting 'txn.user = None' raise ValueError.

[GitHub]

  Branch: refs/heads/restore-100-pct-coverage
  Home:   https://github.com/zopefoundation/transaction
  Commit: 6e976142d2dd86b48bea5ef628826b8dc22d4cc6
      https://github.com/zopefoundation/transaction/commit/6e976142d2dd86b48bea5ef628826b8dc22d4cc6
  Author: Tres Seaver <tseaver at palladion.com>
  Date:   2017-05-05 (Fri, 05 May 2017)

  Changed paths:
    M transaction/_transaction.py
    M transaction/tests/test__transaction.py

  Log Message:
  -----------
  Make setting 'txn.user = None' raise ValueError.