Add type annotations

[Peterl777]

I've just started using pdf2image and am loving it. I'd like to do a PR to add type PEP 484 type hints / type annotations. I have a fair bit of experience with type hints and find them very useful. As it is already marked 3.6+, I don't see there should be any problems with adding hinting. I'm aware this would create many changed lines; does any have any suggestions or comments about managing this?

[Belval]

I would be more than happy to accept a PR that adds type hinting, no need to worry about the number of changed lines, as long as it works.