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/unit/__init__.py

@@ -36,3 +36,16 @@ from .remove_unit import remove_unit
 from .unassign_unit import unassign_unit
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "add_context_dependent_unit",
+    "add_conversion_based_unit",
+    "add_monetary_unit",
+    "add_si_unit",
+    "assign_unit",
+    "edit_derived_unit",
+    "edit_monetary_unit",
+    "edit_named_unit",
+    "remove_unit",
+    "unassign_unit",
+]