py/deactivate vs slots: A slot variable could not be initialized at all

fe2219f4 (On deactivate release in-slots objects too) started to release
objects from slotted variables but was not careful enough while doing so: we
have to be careful while deleting as for unset variables it will raise
AttributeError:

    class C(object):
        __slots__ = ['aaa', 'unset']

        def __init__(self, aaa):
            self.aaa = 1

    c = C(111)
    del c.aaa
    del c.unset

    Traceback (most recent call last):
      File "y.py", line 9, in <module>
        del c.unset
    AttributeError: unset

Caught by preparing fix for https://github.com/zopefoundation/persistent/pull/44#issuecomment-256768600

C version already have this protection in the original fe2219f4 patch:

    +            if (PyObject_GenericSetAttr((PyObject *)self, name, NULL) < 0)
    +                /* delattr of non-set slot will raise AttributeError - we
    +                 * simply ignore. */
    +                PyErr_Clear();

On deactivate release in-slots objects only for classes that don't ov…

Commit fe2219f4 taught Persistent to release in-slots objects on
deactivation. That however broke pure-python implementation of many
things because they were using __slots__ as a place which survive
deactivation.

As per discussion in

    https://github.com/zopefoundation/persistent/pull/44

and resolution:

    https://github.com/zopefoundation/persistent/pull/44#issuecomment-261019084

let's try to preserve backward compatibility by not releasing slots for
classes that override __new__

/proposed-by-and-helped @jimfulton

[ci skip]

Get rid of ctypes dependency to make persistent more pure-python compatible.

Update PR's branch to fix tests under Py3k / pypy3.

- Use 'sys.maxsize' rather than 'sys.maxint' for Py3k compatibility.

- Define it as a module constant, and patch it locally in tests, rather
  than patching 'sys'.

- Call 'c.__hash__()' directly in 'test_hash_equal_constants' to work
  around odd truncation behavior of 'hash()' builtin in pypy3.

[ci skip]

On deactivate release in-slots objects too

On ._p_deactivate() and ._p_invalidate(), when an object goes to ghost
state, objects referenced by all its attributes, except related to
persistence machinery, are released, this way freeing memory (if they
were referenced only from going-to-ghost object).

That's the idea - an object in ghost state is simply a stub, which loads
its content on first access (via hooking into get/set attr) while
occupying minimal memory in not-yet-loaded state.

However the above is not completely true right now, as currently on
ghostification only object's .__dict__ is released, while in-slots objects
are retained attached to ghost object staying in RAM:

    ---- 8< ----
    from ZODB import DB
    from persistent import Persistent
    import gc

    db = DB(None)
    jar = db.open()

    class C:
        def __init__(self, v):
            self.v = v
        def __del__(self):
            print 'released (%s)' % self.v

    class P1(Persistent):
        pass

    class P2(Persistent):
        __slots__ = ('aaa')

    p1 = P1()
    jar.add(p1)
    p1.aaa = C(1)

    p2 = P2()
    jar.add(p2)
    p2.aaa = C(2)

    p1._p_invalidate()
    # "released (1)" is printed

    p2._p_invalidate()
    gc.collect()
    # "released (2)" is NOT printed     <--
    ---- 8< ----

So teach ghostify() & friends to release objects in slots to free-up
memory when an object goes to ghost state.

NOTE PyErr_Occurred() added after ghostify() calls because
pickle_slotnames() can raise an error, but we do not want to change
ghostify() prototype for backward compatibility reason - as it is used
in cPersistenceCAPIstruct.

( I hit this bug with wendelin.core which uses proxies to load
  data from DB to virtual memory manager and then deactivate proxy right
  after load has been completed:

  https://lab.nexedi.com/nexedi/wendelin.core/blob/f7803634/bigfile/file_zodb.py#L239
  https://lab.nexedi.com/nexedi/wendelin.core/blob/f7803634/bigfile/file_zodb.py#L295 )

Fix #38 by returning the correct type from TimeStamp_hash.

CI for travis and appveyor

TimeStamp: sub-second precision for Python implementation of timeTime()

The fractional part of return values was always 0.

Add support for Python 3.5.

Don't use/poison global wheel cache when building w/ PURE_PYTHON.

[ci skip].

Drop support for Python 2.6 and 3.2.

- 2.6 is long out-of-maintenance, and a security quagmire.

- 3.2 cannot be tested on Travis.

[ci skip]

[ci skip]