text: Title: AttributeError: 'WorkerController' object has no attribute 'slaveinput'
  Body: I recently upgraded from pytest-xdist 1.34.0 to 2.0.0 and found that tests always fail with the new version. I pinned the previous version as a workaround.
  
  I've tested on Ubuntu 18.04 and the circle [CI docker image for python 3.7.6](https://github.com/CircleCI-Public/circleci-dockerfiles/blob/master/python/images/3.7.6/Dockerfile)
  
  If I remove --cov=project_folder (project_folder was redacted), the tests run successfully.
  
  ```
  pytest --numprocesses=auto --cov=project_folder test itest
  
  ============================= test session starts ==============================
  platform linux -- Python 3.7.6, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
  rootdir: /root/repo
  plugins: xdist-2.0.0, forked-1.3.0, cov-2.10.0
  gw0 C / gw1 I / gw2 I / gw3 I / gw4 I / gw5 I / gw6 I / gw7 I / gw8 I / gw9 I / gw10 I / gw11 I / gw12 I / gw13 I / gw14 I / gw15 I / gw16 I / gw17 IINTERNALERROR> Traceback (most recent call last):
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/_pytest/main.py", line 238, in wrap_session
  INTERNALERROR>     config.hook.pytest_sessionstart(session=session)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
  INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
  INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
  INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
  INTERNALERROR>     return outcome.get_result()
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
  INTERNALERROR>     raise ex[1].with_traceback(ex[2])
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
  INTERNALERROR>     res = hook_impl.function(*args)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/xdist/dsession.py", line 78, in pytest_sessionstart
  INTERNALERROR>     nodes = self.nodemanager.setup_nodes(putevent=self.queue.put)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/xdist/workermanage.py", line 65, in setup_nodes
  INTERNALERROR>     return [self.setup_node(spec, putevent) for spec in self.specs]
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/xdist/workermanage.py", line 65, in <listcomp>
  INTERNALERROR>     return [self.setup_node(spec, putevent) for spec in self.specs]
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/xdist/workermanage.py", line 73, in setup_node
  INTERNALERROR>     node.setup()
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/xdist/workermanage.py", line 260, in setup
  INTERNALERROR>     self.config.hook.pytest_configure_node(node=self)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
  INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
  INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
  INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
  INTERNALERROR>     return outcome.get_result()
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
  INTERNALERROR>     raise ex[1].with_traceback(ex[2])
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
  INTERNALERROR>     res = hook_impl.function(*args)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pytest_cov/plugin.py", line 239, in pytest_configure_node
  INTERNALERROR>     self.cov_controller.configure_node(node)
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pytest_cov/engine.py", line 274, in configure_node
  INTERNALERROR>     workerinput(node).update({
  INTERNALERROR>   File "/root/repo/venv/lib/python3.7/site-packages/pytest_cov/compat.py", line 42, in fn
  INTERNALERROR>     return getattr(obj, attr, *args)
  INTERNALERROR> AttributeError: 'WorkerController' object has no attribute 'slaveinput'
  ```