Replaced _zope_proxy_proxy.c with a new version.

This one comes from zope.proxy commit 2938c335, version 4.1.2dev.
Point to the correct header.
Update the persistence headers.

Hashes of the files' commits in the persistent repo are as follows:

  - cPersistence.h: 8699c7fc
  - ring.h:         5364296a
  - _compat.h:      3769ede0
Make sure proxy descriptors are not shadowed by the inner's attributes.

This was needed since a change introduced in zope.proxy in 2006,
but the local copy of _zope_proxy_proxy.c was even more ancient.
Remove trailing whitespace.
Make _zope_container_contained.c compile under Py3.
Added tox.ini and a manifest.