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/root/__init__.py

@@ -33,3 +33,10 @@ from .reassign_class import reassign_class
 from .remove_product import remove_product
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "copy_class",
+    "create_entity",
+    "reassign_class",
+    "remove_product",
+]