WIP: Resurrect old Python ring implementation and make the CFFI implementation optional

[dataflake]

This is work in progress.

Current issue: A lot of the ring implementation tests as well as picklecache tests seem to expect specific internals and additional APIs from the CFFI implementation.

[dataflake]

Example tracebacks from local testing:

Error in test test_sweep_of_non_deactivating_object (persistent.tests.test_picklecache.PythonPickleCacheTests)
Traceback (most recent call last):
  File "/Users/jens/src/python/Python-3.10.13/lib/python3.10/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/Users/jens/src/python/Python-3.10.13/lib/python3.10/unittest/case.py", line 591, in run
    self._callTestMethod(testMethod)
  File "/Users/jens/src/python/Python-3.10.13/lib/python3.10/unittest/case.py", line 549, in _callTestMethod
    method()
  File "/Users/jens/src/zope/persistent/.tox/py310/lib/python3.10/site-packages/persistent/tests/test_picklecache.py", line 777, in test_sweep_of_non_deactivating_object
    self.assertEqual(cache.full_sweep(), 0)
  File "/Users/jens/src/zope/persistent/.tox/py310/lib/python3.10/site-packages/persistent/picklecache.py", line 325, in full_sweep
    return self._sweep(0)
  File "/Users/jens/src/zope/persistent/.tox/py310/lib/python3.10/site-packages/persistent/picklecache.py", line 49, in locked
    return f(self, *args, **kwargs)
  File "/Users/jens/src/zope/persistent/.tox/py310/lib/python3.10/site-packages/persistent/picklecache.py", line 440, in _sweep
    for node, value in ring.iteritems():
AttributeError: '_DequeRing' object has no attribute 'iteritems'

Error in test test_update_object_size_estimation_simple (persistent.tests.test_picklecache.PythonPickleCacheTests)
Traceback (most recent call last):
  File "/Users/jens/src/python/Python-3.10.13/lib/python3.10/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/Users/jens/src/python/Python-3.10.13/lib/python3.10/unittest/case.py", line 591, in run
    self._callTestMethod(testMethod)
  File "/Users/jens/src/python/Python-3.10.13/lib/python3.10/unittest/case.py", line 549, in _callTestMethod
    method()
  File "/Users/jens/src/zope/persistent/.tox/py310/lib/python3.10/site-packages/persistent/tests/test_picklecache.py", line 787, in test_update_object_size_estimation_simple
    cache[p._p_oid] = p
  File "/Users/jens/src/zope/persistent/.tox/py310/lib/python3.10/site-packages/persistent/picklecache.py", line 245, in __setitem__
    self.ring.ring_node_for(value)
AttributeError: '_DequeRing' object has no attribute 'ring_node_for'