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/context/__init__.py

@@ -31,3 +31,9 @@ from .edit_context import edit_context
 from .remove_context import remove_context
 
 wrap_usecases(__path__, __name__)
+
+__all__ = [
+    "add_context",
+    "edit_context",
+    "remove_context",
+]