Previously, a tool run involved creating the Tool subclass and then

calling a handful of methods on it, each time passing in a settings

argument.
This change simplifies this, instead having the new BaseTool take

settings on construction, so that any method can access it. That

avoids more complicated method signatures going forward.
This in turn deprecates the settings argument on all these methods.

To help transition to this, and avoid issues in the future, these

methods are now all expected to take a **kwargs argument. This allows

us to continue passing in settings for now, and for older tools to

support it, but for newer ones to ignore it.
To transition people to the new behavior, legacy Tool and

RepositoryTool construction (via __new__) now emits a deprecation

warning telling tool authors what must be done to transition their tool.

We're using dedicated deprecation classes in a reviewbot.deprecation

module to ease this and future deprecations, the same way we do with

Review Board and Djblets.
All of our current tools are considered deprecated, and will emit a

warning. Those will be updated one-by-one.

Unit tests pass on Python 2.7 and 3.x.
Ran the worker, refreshed tools, and triggered new code reviews, without

any issues. Saw the deprecation warnings for our tools on launch.