This updates the typing for the hosting service HTTP support, updating

some legacy documentation and bad assumptions in the code accordingly.

This includes the removal of a lot of Python 2-isms in the code and

documentation that are no longer needed.
Functionality marked as deprecated in Review Board 4 have now been given

a deprecation warning, scheduled for removal in Review Board 7.

Unit tests pass.
Verified that pyright and mypy were happy with this module.