specify exported api methods in __all__

It's py.types requirement, similar to 6e2edbf

Example issue without __all__:
import ifcopenshell
import ifcopenshell.api.project

# "create_file" is not exported from module "ifcopenshell.api.project"
ifcopenshell.api.project.create_file()

src/ifcopenshell-python/ifcopenshell/api/aggregate/__init__.py

@@ -28,3 +28,8 @@ from .assign_object import assign_object
 from .unassign_object import unassign_object
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "assign_object",
+    "unassign_object",
+]