The GitHub backend has been wrapping the legacy JSON HTTP functions for

years now. Review Board 4.0 introduced new HTTP functionality that

standardizes error handling, post-request handling, header access, and

more, which was largely based off the needs from the GitHub support.

Until now, though, GitHub has retained its own implementation.
This change redoes the GitHub code to use the modern HTTP support. For

compatibility, some legacy functions (api_get(), api_post(), and

api_delete()) remain as deprecated wrappers, but all internal code is

now updated to use the standard HTTP functions. This will make the code

more maintainable going forward.

Unit tests pass on Python 2.7, 3.6, 3.7, and 3.8.