Consider adding type hints

[bgilbert]

Type hints are now available in every Python version supported by OpenSlide Python. Consider adding them at least to the public API.

[bgilbert]

WIP in #164.

[bgilbert]

Hints added in #260. We still need py.typed, hints for tests and examples, and updated docs. PRs will be coming in the next few days.

[bgilbert]

Done in #281, #260, #286, and #292.