GitHubAuth fails under Python 3.4

[zware]

Received this traceback in the log after clicking the 'Login via GitHub' link and authorizing the app:

2017-06-25 04:02:00+0000 [-] While rendering resource:
        Traceback (most recent call last):
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 1442, in gotResult
            _inlineCallbacks(r, g, deferred)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 1432, in _inlineCallbacks
            deferred.errback()
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 500, in errback
            self._startRunCallbacks(fail)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 567, in _startRunCallbacks
            self._runCallbacks()
        --- <exception caught here> ---
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 653, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/buildbot/www/resource.py", line 91, in failHttpError
            f.trap(Error)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/failure.py", line 346, in trap
            self.raiseException()
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/failure.py", line 372, in raiseException
            raise self.value.with_traceback(self.tb)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 653, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/buildbot/www/resource.py", line 84, in failHttpRedirect
            f.trap(Redirect)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/failure.py", line 346, in trap
            self.raiseException()
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/failure.py", line 372, in raiseException
            raise self.value.with_traceback(self.tb)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/failure.py", line 393, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/buildbot/www/oauth2.py", line 55, in renderLogin
            details = yield self.auth.verifyCode(code)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/buildbot/www/oauth2.py", line 163, in thd
            session = self.createSessionFromToken(content)
          File "/data/buildbot/test-master/venv/lib/python3.4/site-packages/buildbot/www/oauth2.py", line 119, in createSessionFromToken
            s.params = {'access_token': token['access_token']}
        builtins.TypeError: byte indices must be integers, not str

Python: 3.4.3 (Ubuntu 14.04 system python3; running from a venv)
Buildbot: 0.9.8 (all deps latest)
Configuration: this

Works fine (as far as I can tell; first time I've tried it) under Python 2.7.