Add typing

[ZivRonen]

I would like to add typing support to this library.
However, as multiple changes will be required to support this, I wanted to verify that it is something that is wanted before I start to do so.

Note that it will require future code to support typing as well. If typing is wanted, but you don't want to change the code base here or promise future support, I can add it to typeshed instead.

[phi-friday]

#202 (comment)

He doesn't seem to be willing to support typing, at least for the time being.
I also needed type hints, so I created a related library.

[CNSeniorious000]

Have a try of diskcache2, which is fork of diskcache that only added some type hints without modifying any API or logic.