BTrees and pop()

Yesterday Tim Peters merged my branch for pop() method in
BTree and Bucket into main trunk, so ZODB3.6 now has this
feature and is closer to what python’s mapping protocol provides.

Of course Tim made modifications to C code to make it more simple
and added more tests Thanks Tim!

Now we have both setdefault() and pop() methods,
hope people will find it useful in their ZODB applications.

(Post originally written by Ruslan Spivak on the old Nuxeo blogs.)