Improve type-hinting

[acampove]

Hi,

I am having the same problem as:

#2542

and I see that that issue is closed so I will open a new one here. In particular I am seeing:

     class CompareTask(law.Task):                                                                                                                                                   
E    config_string : str = Parameter()     ■ Type "Parameter" is not assignable to declared type "str"    "Parameter" is not assignable to "str"