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/group/__init__.py

@@ -32,3 +32,12 @@ from .unassign_group import unassign_group
 from .update_group_products import update_group_products
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "add_group",
+    "assign_group",
+    "edit_group",
+    "remove_group",
+    "unassign_group",
+    "update_group_products",
+]