[mypyc] Fix `__init_subclass__` running before `ClassVar` instantiations (#20916)
Fixes https://github.com/mypyc/mypyc/issues/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:
```Python3
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`
mypy
f3e2a977 - [mypyc] Fix `__init_subclass__` running before `ClassVar` instantiations (#20916)
