* Fix type declarations for multiprocessing.pool.ThreadPool
The existing incomplete type declarations were replaced by a copy of
multiprocessing.Pool's declarations, suitably modified.
Fixes#683.
* Change iterable type to Iterable[Iterable[Any]] in starmap
multiprocessing.Pool and multiprocessing.pool.ThreadPool have starmap
and related methods, where the iterable argument is expected to yield
iterables. The type declarations now reflect this.