The Mercurial hgweb backend had a couple of compatibility problems that

prevented it from working on Python 3 and Review Board 4.0. It was

trying to parse JSON payloads in the form of byte strings, which is not

supported on Python 3. Instead, we needed to ensure we were providing

Unicode strings. A handy new _get_http_json() method now does the work

of ensuring we're using the correct types.
When building commits, it was still passing a base_commit_id=

parameter to the Commit constructor, which has been removed in

Review Board 4.0. This was missed before due to the lack of unit tests

for the hgweb support.
Since we now have a _get_http_json() method that handles JSON API

requests in a more standardized way, we're also in a better position to

future-proof by checking if endpoints are implemented (comparing a payload

to b'not yet implemented') and then returning None. This ensures

we don't break with older versions of Mercurial now, and that going

forward we can introduce support for newer APIs without outright breaking

on version that lack them.
To ensure we have working compatibility now and in the future, this

change also adds unit tests that cover the hgweb communication.

Manually tested loading branches and commits, posting a change for

review, and viewing a diff.
Unit tests pass on all supported versions of Python.