CI is broken on Python 3.5

[eric-wieser]

Doesn't look like our fault at all. Failures all look something like this

internal error
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/site-packages/_pytest/main.py", line 240, in wrap_session
    session.exitstatus = doit(config, session) or 0
  File "/usr/local/lib/python3.5/site-packages/_pytest/main.py", line 296, in _main
    config.hook.pytest_runtestloop(session=session)
  File "/usr/local/lib/python3.5/site-packages/pluggy/hooks.py", line 286, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "/usr/local/lib/python3.5/site-packages/pluggy/manager.py", line 93, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/usr/local/lib/python3.5/site-packages/pluggy/manager.py", line 87, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "/usr/local/lib/python3.5/site-packages/pluggy/callers.py", line 203, in _multicall
    gen.send(outcome)
  File "/usr/local/lib/python3.5/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/usr/local/lib/python3.5/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "/usr/local/lib/python3.5/site-packages/xdist/dsession.py", line 112, in pytest_runtestloop
    self.loop_once()
  File "/usr/local/lib/python3.5/site-packages/xdist/dsession.py", line 135, in loop_once
    call(**kwargs)
  File "/usr/local/lib/python3.5/site-packages/xdist/dsession.py", line 241, in worker_collectionfinish
    self.sched.schedule()
  File "/usr/local/lib/python3.5/site-packages/xdist/scheduler/loadscope.py", line 371, in schedule
    self._assign_work_unit(node)
  File "/usr/local/lib/python3.5/site-packages/xdist/scheduler/loadscope.py", line 265, in _assign_work_unit
    node.send_runtest_some(nodeids_indexes)
  File "/usr/local/lib/python3.5/site-packages/xdist/workermanage.py", line 290, in send_runtest_some
    self.sendcommand("runtests", indices=indices)
  File "/usr/local/lib/python3.5/site-packages/xdist/workermanage.py", line 306, in sendcommand
    self.channel.send((name, kwargs))
  File "/usr/local/lib/python3.5/site-packages/execnet/gateway_base.py", line 728, in send
    raise IOError("cannot send to {!r}".format(self))
OSError: cannot send to <Channel id=1 closed>

(example: https://app.circleci.com/pipelines/github/pygae/galgebra/1135/workflows/3d6ad7e9-3b1e-491b-99c4-168d536539fa/jobs/6559)