This introduces type hints for some of our most important underlying

structures in Review Board. Types are now available for SCMTool,

SCMClient, Branch, Commit, ChangeSet, Revision,

FileLookupContext, and _SCMToolIDProperty.
This should help ensure proper usage of these classes, and proper

implementation of new SCMTools.
Some typing errors and and bad documentation were uncovered while

adding types. For instance, Popen had a bad string conversion and

bad documented return type, and some objects had unsafe comparison

logic. These have been fixed.

Unit tests pass.