tox-pipenv: tox 3.8.1 breaks us

To reproduce:

docker run --rm -it -v $(pwd):$(pwd) -w $(pwd) python:3.7-slim /bin/bash
pip3 install -U tox tox-pipenv
tox

Workaround, fall back to tox 3.7.0:

pip3 install tox==3.7.0

Error:

Traceback (most recent call last):
  File "/usr/local/bin/tox", line 10, in <module>
    sys.exit(cmdline())
  File "/usr/local/lib/python3.7/site-packages/tox/session/__init__.py", line 42, in cmdline
    main(args)
  File "/usr/local/lib/python3.7/site-packages/tox/session/__init__.py", line 63, in main
    retcode = session.runcommand()
  File "/usr/local/lib/python3.7/site-packages/tox/session/__init__.py", line 187, in runcommand
    return self.subcommand_test()
  File "/usr/local/lib/python3.7/site-packages/tox/session/__init__.py", line 215, in subcommand_test
    run_sequential(self.config, self.venv_dict)
  File "/usr/local/lib/python3.7/site-packages/tox/session/commands/run/sequential.py", line 9, in run_sequential
    if venv.setupenv():
  File "/usr/local/lib/python3.7/site-packages/tox/venv.py", line 584, in setupenv
    status = self.update(action=action)
  File "/usr/local/lib/python3.7/site-packages/tox/venv.py", line 242, in update
    self.hook.tox_testenv_create(action=action, venv=self)
  File "/usr/local/lib/python3.7/site-packages/pluggy/hooks.py", line 289, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "/usr/local/lib/python3.7/site-packages/pluggy/manager.py", line 68, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/usr/local/lib/python3.7/site-packages/pluggy/manager.py", line 62, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "/usr/local/lib/python3.7/site-packages/pluggy/callers.py", line 208, in _multicall
    return outcome.get_result()
  File "/usr/local/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/usr/local/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "/usr/local/lib/python3.7/site-packages/tox_pipenv/plugin.py", line 64, in tox_testenv_create
    venv.session.make_emptydir(venv.path)
AttributeError: 'VirtualEnv' object has no attribute 'session'