Branch scanner jobs failing in bzrlib

We've been seeing failures like OOPS-1863SSMS1 in the branch scanner, as well as the translations branch approver.

  ImportError: No module named http._urllib

Traceback (most recent call last):
  Module lp.services.scripts.base, line 91, in log_unhandled_exceptions_func
    return func(self, *args, **kw)
  Module lp.services.scripts.base, line 353, in lock_and_run
    implicit_begin=implicit_begin, isolation=isolation)
  Module lp.services.scripts.base, line 91, in log_unhandled_exceptions_func
    return func(self, *args, **kw)
  Module lp.services.scripts.base, line 311, in run
    self.main()
  Module lp.services.job.runner, line 480, in main
    job_source, self.dbuser, self.logger)
  Module lp.services.job.runner, line 255, in runFromSource
    with removeSecurityProxy(job_source.contextManager()):
  Module contextlib, line 16, in __enter__
    return self.gen.next()
  Module lp.code.model.branchjob, line 334, in contextManager
    cls.server = get_ro_server()
  Module lp.codehosting.vfs.branchfs, line 206, in get_ro_server
    get_transport(config.codehosting.internal_branch_by_id_root))
  Module bzrlib.transport, line 1602, in get_transport
  Module bzrlib.transport, line 1625, in _try_transport_factories
  Module bzrlib.registry, line 61, in get_obj
  Module bzrlib.registry, line 70, in _do_import
ImportError: No module named http._urllib