This adds type safety to important attributes on Branch, ChangeSet,

Commit, and Revision, forcing hosting services and SCMTools to

encode or decode data properly and pass the right types during

construction. This allows consumers of these objects to be certain about

what type they're working with, and just keeps everything nice and

consistent across implementations and Python versions.
The new type safety caught some issues in a few of our backends, where

we were passing Unicode diffs or byte string metadata. These haven't

been a problem before (and we had some backwards-compatibility support

in Commit that dealt with Unicode diffs, which are now gone). These

cases have all been updated to pass the correct types.

Unit tests pass on Python 2.7 and 3.7 (with other in-progress changes).
Tested browsing for commits and posting commits for review using GitHub,

Subversion, and Perforce (the three most heavily changed), complete with

diffs using Emoji characters.