The upgrade step is breaking on Python 3.6. See https://travis-ci.org/zopefoundation/zope.security/jobs/270003178

Fixes #27.

Add special cases to defaultCheckers for the two types of objects that
can be returned from zope.interface.providedBy. On Python 2, these
were never proxied, but on Python 3 they were. Now it's
consistent (they're never proxied). (Using an _iteratorChecker for
them would be a breaking change because the results of iterating them
would be security proxied interface objects that don't compare
equally.)

Also fix `__length_hint__` while we're at it. Previously it was
ignored because it is looked up on the type of the object, and proxy
didn't implement that. So implement it, and add it to the list of
names allowed for iterators.

OrderedDict, BTree and dict all iterate the same way.

Fixes #23. Also a further fix for #20 (you couldn't iterate a BTree
all by itself).

Refactor the test case for BTree to be a shared implementation and
confirm that it works as expected for dict, using the actual dict
checker. Then apply it to OrderedDict and BTree and fix the resulting
failures by refactoring the fixup in checker.py to a shared
implementation and applying it.

Be specific that BTrees.keys and .values also are fixed