execnet.gateway_base.DumpError: can't serialize <class '_pytest.warning_types.RemovedInPytest4Warning'>

Stumbled upon this with borg's tests: https://travis-ci.org/borgbackup/borg/jobs/439508853#L2123

  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1383, in _save
    dispatch = self._dispatch[tp]
KeyError: <class '_pytest.warning_types.RemovedInPytest4Warning'>
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1072, in executetask
    do_exec(co, loc)  # noqa
  File "<string>", line 1, in do_exec
  File "<remote exec>", line 261, in <module>
  File "<remote exec>", line 25, in __init__
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/_pytest/config/__init__.py", line 303, in register
    ret = super(PytestPluginManager, self).register(plugin, name)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/manager.py", line 101, in register
    hook._maybe_apply_history(hookimpl)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/hooks.py", line 331, in _maybe_apply_history
    res = self._hookexec(self, [method], kwargs)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/manager.py", line 67, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/manager.py", line 61, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/callers.py", line 208, in _multicall
    return outcome.get_result()
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "<remote exec>", line 135, in pytest_warning_captured
  File "<remote exec>", line 29, in sendevent
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 717, in send
    self.gateway._send(Message.CHANNEL_DATA, self.id, dumps_internal(item))
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1354, in dumps_internal
    return _Serializer().save(obj)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1372, in save
    self._save(obj)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1390, in _save
    dispatch(self, obj)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1475, in save_tuple
    self._save(item)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1390, in _save
    dispatch(self, obj)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1471, in save_dict
    self._write_setitem(key, value)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1465, in _write_setitem
    self._save(value)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1390, in _save
    dispatch(self, obj)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1471, in save_dict
    self._write_setitem(key, value)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1465, in _write_setitem
    self._save(value)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1390, in _save
    dispatch(self, obj)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1475, in save_tuple
    self._save(item)
  File "/Users/travis/build/borgbackup/borg/.tox/py34/lib/python3.4/site-packages/execnet/gateway_base.py", line 1388, in _save
    raise DumpError("can't serialize %s" % (tp,))
execnet.gateway_base.DumpError: can't serialize <class '_pytest.warning_types.RemovedInPytest4Warning'>
Replacing crashed worker gw0

Python 3.4.5, pytest-3.9.3, py-1.7.0, pluggy-0.8.0

Issue: https://github.com/borgbackup/borg/issues/4123

The traceback points to execnet/gateway_base.py, especially _Serializer.save/_save and channel send; begin by reproducing the Borg test failure with Python 3.4, pytest 3.9.3, and the listed dependencies. Done means the warning event crosses the worker channel without DumpError or worker replacement.