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/constraint/__init__.py

@@ -34,3 +34,15 @@ from .remove_metric import remove_metric
 from .unassign_constraint import unassign_constraint
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "add_metric",
+    "add_metric_reference",
+    "add_objective",
+    "assign_constraint",
+    "edit_metric",
+    "edit_objective",
+    "remove_constraint",
+    "remove_metric",
+    "unassign_constraint",
+]