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/classification/__init__.py

@@ -36,3 +36,12 @@ from .remove_classification import remove_classification
 from .remove_reference import remove_reference
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "add_classification",
+    "add_reference",
+    "edit_classification",
+    "edit_reference",
+    "remove_classification",
+    "remove_reference",
+]