The DVCS-specific API resources were incorrectly using byte strings for

their method names. On Python 2, this would result in sending

HTTP POST, but on Python 3 we would send HTTP b'POST' and end up

getting HTTP 400 errors back. If we use the text type, it works on both

Python 2 and Python 3.

Confirmed the following on Python versions 2.7, 3.5, 3.6, and 3.7:

With this entire patch stack (/r/10685, /r/10696, and /r/10697),

rbt post is able to create review requests with commit history.