Suggestion: use concurrent.futures for multithreading

[yprez]

Since newspaper is now Python 3 only, concurrent.futures can be used in mthreading.py instead of raw access to threads.

For example, the pool management and such (like here: https://github.com/codelucas/newspaper/blob/master/newspaper/mthreading.py#L45) already exists there, and there's no reason to re-implement features that already exist in Python (3).

Also: less code => less bugs.

Maybe fix/enable some of the mutlithreading tests on the way, I've noticed these have been disabled for some reason.