ContainedProxy providedBy caching broken in 4.4.0

[jamadden]

After 1523b1c, it is no longer necessarily the case that c = ContainedProxy(obj); providedBy(c) is providedBy(c); previously it was.

Whether or not it is true depends on whether obj provides interfaces that overlap with ContainedProxy and which thus have to be removed.

Provides(cls, provided) in zope.interface does caching using (cls, provided) as the (strongly referenced) cache key. But the descriptor used to implement __provides__ for ContainedProxy calculates provided as a new object: provided = provided - duplicate_interface, and hashing of that object is based on identity.

(If someone keeps alive a reference to providedBy(c), it will also stick around uselessly forever. )

[jamadden]

Example:

>>> from zope.interface import *
>>> from zope.container.contained import ContainedProxy
>>> from persistent import Persistent

>>> class Foo:  # No overlap
…    pass
>>> c = ContainedProxy(Foo())
>>> providedBy(c) is providedBy(c)
True

>>> c = ContainedProxy(Persistent()) # These overlap
>>> providedBy(c) is providedBy(c)
False