CLN: replace Dict with Mapping to annotate arguments
xref https://docs.python.org/3/library/typing.html#typing.Dict

cc @WillAyd 

> lgtm merge on green.

shows green on Travis, github shows in progress. will merge.


> Not sure if overkill but note in contributing guide may be useful as well for follow up

i'll do similar changes for List first, xref https://docs.python.org/3/library/typing.html#typing.List

probably go with Iterable, and use Sequence where needed.

see how that goes and then update guide accordingly. > probably go with Iterable, and use Sequence where needed.

Both of these would allow strings then right? Might be less universally applicable to go from List to one of these > > probably go with Iterable, and use Sequence where needed.
> 
> Both of these would allow strings then right? Might be less universally applicable to go from List to one of these

if these are typed as generic, then maybe so, but not if being precise. 
(we should probably have `disallow_any_generics`  in our config.)