[Checkins] [zopefoundation/persistent] 2492a5: Add the ability to force use of C extensions.

[Jason Madden]

  Branch: refs/heads/issue131
  Home:   https://github.com/zopefoundation/persistent
  Commit: 2492a5cf66a1350d9b68eb2e5813fe6529ded84c
      https://github.com/zopefoundation/persistent/commit/2492a5cf66a1350d9b68eb2e5813fe6529ded84c
  Author: Jason Madden <jamadden at gmail.com>
  Date:   2020-02-20 (Thu, 20 Feb 2020)

  Changed paths:
    M .travis.yml
    M CHANGES.rst
    M persistent/__init__.py
    M persistent/_compat.py
    M persistent/cPersistence.c
    M persistent/persistence.py
    M persistent/picklecache.py
    M persistent/ring.py
    M persistent/tests/test_persistence.py
    M persistent/tests/test_picklecache.py
    M persistent/tests/test_timestamp.py
    M persistent/tests/utils.py
    M persistent/timestamp.py
    M setup.py
    M tox.ini

  Log Message:
  -----------
  Add the ability to force use of C extensions.

With PURE_PYTHON=0, like in zope.interface.

Also always require all three extensions. This solves mysterious issues you can get if you wind up mixing and matching (#124).

Fixes #131

Add travis and tox tests for this.