Ok, I'll give another try at rectifying these issues throughout all the files.
Just a quick question, did you review the changes for all the files, or did you review the changes only up to base.py and didn't review further because of the repeated issues?

@ardumont : Ok, removed the suffix "Closes T2259" from DIff title and summary.

Change type annotations to more concrete type
Change Type annotations for all files in swh.indexer.metadata_mapping to reflect more concrete type annotations.
Remove unncesseary usage of # type: ignore

Thanks for a prompt review.
I'll go level deeper into annotating wherever I have used Any, and wherever it's possible to go a level deeper. Will submit an updated diff by tomorrow.