add type annotations to client.dynamic

[junkmd]

THIS IS FOR drop_py2 PLAN! Please see #392.

It can be only a part of the codebase, but a module as size as `client.dynamic' can probably do it all at once.

Some of them may be difficult to clearly define type annotations for.
In that case, please do not change that part and write the reason in the PR kanban.
The maintainer or this issue assignee should submit a separate issue for that task only.

[junkmd]

I created this issue to serve as a "model case" for future good first issue that I will submit.

I will work on this.