This updates reviewboard.diffviewer.parser to include full type hints,

making it easier to properly implement a diff parser or change diff

parsing logic. This is being done in preparation for additional work for

diff parsers.
Several methods have been updated to require keyword-only arguments,

deprecating positional arguments.
We also had some parameters that were meant to be removed back in

Review Board 6. We never added deprecation warnings for those changes,

so those have been added, and the deprecations have been pushed to

Review Board 9.

Unit tests pass.