Adding type hints

[duynhaaa]

As mentioned in Pyramid's issue #2638, I would like to start adding type annotations to WebOb.

Since I don't really know the code base really well (I'm learning on the go!), I'm planning to make many little PRs so that they are easily reviewed and get feedbacks quickly (also because they should not affect runtime at all).

Can I get a green light on this?

Also, is it okay to make use of this issue to track progress? Google group is too noisy for my liking.