Also, if one have in mind that dict addition with the `+` operator had been
recelently approved, as of Python 3.9 it will be ok to write:
```
return ({
'user_id': user_id,} +
({'max_results': max_results} if max_results else {}) +
({'active': active} if active is not None else {}) +
({'deleted': deleted} if deleted is not None else {})
)
And actually, on a second though, this is already valid as of current Python - and is potentially more
readable than a magic "dict.skip" singleton.
```
return {
'user_id': user_id,
**({'max_results': max_results} if max_results else {}),
**({'active': active} if active is not None else {}),
**({'deleted': deleted} if deleted is not None else {})
}
```