This introduces a new djblets.deprecation class, which currently

defines DeprecationWarning subclasses for Djblets, and a utility

function for defining warn-on-access deprecated arguments for functions,

signal handlers, etc. The warnings are the meat of this change, and

include a base class, BaseRemovedInDjbletsVersionWarning, a

subclass specific to Djblets 2.0, RemovedInDjblets20Warning, and

an alias for the version-specific warning,

RemovedInNextDjbletsVersionWarning.
These allow us to specify when exactly a particular feature will be

removed, so it's clear to consumers and so we can easily locate them

when getting ready to remove features. It also helps keep us consistent

with when we actually remove deprecated features.
All current DeprecationWarnings have been updated to use the 2.0

warning. Some of the messages referred to Djblets 0.9 or 1.0, and we

clearly never got rid of that code for those versions, so those have

been updated to reference 2.0, where they'll actually be removed.

All unit tests pass.