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In zope.interface, you have something the following construct:
class InterfaceBase:
 pass
Interface = InterfaceBase()
Using the above Interface as a derivation base for your own classes, will make that instance a type derived class:
class IFoo(Interface):
 pass
type(IFoo)
-> Interface
type(Interface)
-> type
I wonder why this behavior is not documented in the official documentation, or at least, I was unable to find it there...

This doesn't work as you show. What you probably meant was something like this:
class InterfaceBase(type):
 ...
Interface = InterfaceBase('Interface', (), {})
class IFoo(Interface):
 ...
which you can just as well do by using normal metaclass syntax.