Inside rqltypes module
- define a protocol to describe an object that:
  - has an attribute named `parent`
  - parent attribute should have an attribute named `relation`
  - a method named `get_type`

Inside HSMixin:
- remove now useless type:ignore comments
- type `self` as a `HasHand` type

Inside nodes.Constant `get_type` method could return a str or None:
- make sure `get_type` always returns a str
- change the default value of `_uidtype` from None to empty string
- type `_uidtype` as a str