The Gerrit hosting service had some compatibility issues with Python 3,

and this was due to a queryset ordering discrepency issue that was

solved in Review Board 4.0's hosting service client rewrite.
Rather than take the easy route and apply the same fix to Gerrit's code,

this change updates the Gerrit codebase to make full use of the modern

client support. We now use the request/response capabilities in that

rewrite to better control how authentication is performed and to use the

standard methods for performing HTTP requests, allowing us to remove the

custom api_* request methods.
Since Gerrit's API has a security-related quirk where it prefixes JSON

payloads with a string we have to filter out, we still needed a way to

strip this out in a standard way. Previously, we overrode a private

method in HostingServiceClient to do this, which wasn't future-proof.

Now, as part of this change, a HostingServiceClient can designate a

subclass of HostingServiceHTTPResponse to use for all responses, which

the Gerrit implementation uses in order to strip out this prefix when

requesting a JSON-deserialized response.

Manually tested linking a new repository and account, viewing branches,

listing commits, and posting commits for review.
Unit tests pass.