[mypy] Add more types

Hello dear Pythonistas, the following is an attempt to add more types within the following modules:

• rql/interfaces.py
• rql/rqlgen.py
• rql/utils

It (interfaces.py) could be kept because:

• the intent seems to be: defining an interface to enforce
• we could use it to define protocols for example: IsSomething, HasSomething, ...

It could have been deleted because:

• it seems it is not used
• logilab.common.interfaces will be deprecated

Thanks a lot for your help, advices and suggestions.