Add type hints to DQN and its dependencies

[muupan]

depends on #16

Adding type hints required some code changes as well. I think these are good changes.

• Define ActionValue.__getitem__
• Define AbstractReplayBuffer.capacity
• Move stop_current_episode from AbstractEpisodicReplayBuffer to AbstractReplayBuffer.
• Before this PR, self.target_model is initialized as None and assigned by sync_target_network later. After this PR, it cannot be None anymore.

[ummavi]

Thanks for the PR! A general recommendation I want to discuss is collapsing the from typing ... imports into a single line (or few lines). Especially for the dqn.py which is already quite sizable.
Since we've done this a few other places I suppose we don't have any style guides against it.

[muupan]

Fixed conflicts.

• update_counter's type would be multiprocessing.sharedctypes.Synchronized, but I could not specify the type due to this issue: Stub missing for multiprocessing.sharedctypes python/typeshed#4047. Thus it is Any.
• selt.target_q_function is removed since it is not used anywhere.

We agreed that we will introduce isort, which automatically merge import sentences. It will be addressed by #53.

[ummavi]

Thanks! LGTM.