Regression in 4.9: Subclasses can't use @adapter

[jamadden]

Because of the custom metaclass. Previously this code used to work:

>>> import BTrees
>>> from zope.component import adapter
>>> from zope.interface import Interface
>>> @adapter(Interface)
... class Subclass(BTrees.family64.OO.BTree):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "//zope/component/_declaration.py", line 33, in __call__
    ob.__component_adapts__ = _adapts_descr(self.interfaces)
TypeError: BTrees: cannot set attribute '__component_adapts__' of immutable type 'Subclass'

Apparently there exists code like this in the wild.