the base classes is not a classic class, and its class (the metaclass)
is callable, call the metaclass to do the deed.

One effect of this is that, when mixing classic and new-style classes
amongst the bases of a class, it doesn't matter whether the first base
class is a classic class or not: you will always get the error
"TypeError: metatype conflict among bases".  (Formerly, with a classic
class first, you'd get "TypeError: PyClass_New: base must be a class".)

Another effect is that multiple inheritance from ExtensionClass.Base,
with a classic class as the first class, transfers control to the
ExtensionClass.Base class.  This is what we need for SF #443239 (and
also for running Zope under 2.2a4, before ExtensionClass is replaced).