weakref.WeakKeyDictionary and weakref.WeakValueDictionary

* Check the found object for a None value during a contains/has_key
  lookup.  Perhaps it will help the OP who is likely suffering from an
  occassional GC or threading object deletion after self.data is checked.

* Complete the previous patch by removing the unnecessary indirection
  for weak dict iterators.  Makes the code cleaner and more readable.

- weakref.ref and weakref.ReferenceType will become aliases for each
  other

- weakref.ref will be a modern, new-style class with proper __new__
  and __init__ methods

- weakref.WeakValueDictionary will have a lighter memory footprint,
  using a new weakref.ref subclass to associate the key with the
  value, allowing us to have only a single object of overhead for each
  dictionary entry (currently, there are 3 objects of overhead per
  entry: a weakref to the value, a weakref to the dictionary, and a
  function object used as a weakref callback; the weakref to the
  dictionary could be avoided without this change)

- a new macro, PyWeakref_CheckRefExact(), will be added

- PyWeakref_CheckRef() will check for subclasses of weakref.ref

This closes SF patch #983019.

Add this default to weakref.WeakValueDictionary.setdefault()
and weakref.WeakKeyDictionary.setdefault() too.

(Championed by Bob Ippolito.)

The update() method for mappings now accepts all the same argument forms
as the dict() constructor.  This includes item lists and/or keyword
arguments.

Someone review this, please!  Final releases are getting close, Fred
(the weakref guy) won't be around until Tuesday, and the pre-patch
code can indeed raise spurious RuntimeErrors in the presence of
threads or mutating comparison functions.

See the bug report for my confusions:  I can't see any reason for why
__delitem__ iterated over the keys.  The new one-liner implementation
is much faster, can't raise RuntimeError, and should be better-behaved
in all respects wrt threads.

New tests test_weak_keyed_bad_delitem and
test_weak_keyed_cascading_deletes fail before this patch.

Bugfix candidate for 2.2.3 too, if someone else agrees with this patch.

* Adds missing pop() methods to weakref.py
* Expands test suite to broaden coverage of objects with
  a mapping interface.

Contributed by Sebastien Keim.