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Let the C SecurityManager take part in garbage collection (#178)

* Let the C SecurityManager take part in garbage collection

The C SecurityManager keeps its context, policy and cached bound policy
methods in struct fields, but the type had no tp_traverse and no
tp_clear.  Reference cycles through a security manager were therefore
never collected and leaked everything reachable from them.

The type was declared "old-style" for ExtensionClass (tp_flags == 0),
where the tp_traverse and tp_clear slots carry the method chain and the
ExtensionClass flags.  Declare it "new-style" instead, with
Py_TPFLAGS_HAVE_GC, a tp_traverse and a tp_clear, and untrack the object
in tp_dealloc.  PyExtensionClass_Export_ then installs PyType_GenericNew,
which does not INCREF the type per instance, so the matching DECREF in
tp_dealloc goes away.

Fixes #177

Assisted-by: Claude Fable 5.1

* Drop the Py_TPFLAGS_HAVE_GC constant from the tests

gc.is_tracked() is only true for instances of types that have the
flag, so the flag check duplicated the tracking assertion with a
copied CPython-internal constant.

Assisted-by: Claude Fable 5.1

* Drop the redundant tp_new assignment

PyExtensionClass_Export_ installs PyType_GenericNew when tp_new is
NULL, as the comment above the type says.

Assisted-by: Claude Fable 5.1

* Report a missing security context as _context

CHECK_SECURITY_MANAGER_STATE raised AttributeError('_policy') for a
missing context, a copy-and-paste slip.

Assisted-by: Claude Fable 5.1

* Store new values before releasing old ones in SecurityManager_setattro

ASSIGN released the old context, policy or thread id before storing
the new one, and the _policy branch released the cached bound policy
methods before clearing the fields.  Code that runs while the old
object is torn down, for example a __del__, could reach the dying
object through the manager.  Use Py_XSETREF and Py_CLEAR, which store
first and release afterwards.

Assisted-by: Claude Fable 5.1

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