Add type annotations

[g-k]

We could:

• land the external API stubs in typeshed: https://github.com/python/typeshed/search?q=bleach&unscoped_q=bleach
• add type annotations to internal APIs to catch errors and improve docs
• run typechecks with mypy or pyre in CI (edit: prefer mypy since adding ocaml to the dev env would probably reduce contributions)

Annotations would have to be in comment form to maintain backwards compatibility with Python 2, but we can skip that if #520 lands first.

[mastizada]

In 2 month and 15 days Python2 will be retired and using it will be a security issue by itself (https://pythonclock.org/). I think it will be ok to use Python 3 only annotation after this period.

[willkg]

We deprecated this project, so we're not going to implement more features.