[mypyc] Fix __init_subclass__ running before ClassVar instantiations

[VaggelisD]

Fixes mypyc/mypyc#1086

We also ran into this in SQLGlot; To compile our AST we swapped any metaclasses for __init_subclass__ in which we dynamically set some attributes in it like so:

  class Expression:                                                                                                                                                                                                                                  
      arg_types: ClassVar[...] = {}                                                                                                                                                                                                      
                                                            
      def __init_subclass__(cls, **kwargs: object) -> None:                                                                                                                                                                                          
          cls.required_args = {k for k, v in cls.arg_types.items() if v}

  class Select(Expression):
      arg_types: ClassVar[...] = {...}

I introduced the InitSubclass primitive to allow for fine-grained control of when this gets called and moved it from ExtClassBuilder::__init__ (i.e allocate_class > CPyType_FromTemplate) to ExtClassBuilder::finalize