pipenvとpyenvの共存でエラーが出た時の対処

## はじめに
`pyenv install 3.6.8; pipenv --python 3.6.8`としたら、こんなエラーが出ました。環境はMacOSX HighSierraにて。

``` 
$ pipenv --python 3.6.8
Traceback (most recent call last):
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/python.py", line 112, in get_versions
    version = PythonVersion.parse(p.name)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/python.py", line 359, in parse
    version_dict = parse_python_version(str(version))
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/utils.py", line 86, in parse_python_version
    raise InvalidPythonVersion("%s is not a python version" % version_str)
pipenv.vendor.pythonfinder.exceptions.InvalidPythonVersion: .DS_Store is not a python version

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/bin/pipenv", line 11, in <module>
    load_entry_point('pipenv==2018.11.26', 'console_scripts', 'pipenv')()
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 1114, in invoke
    return Command.invoke(self, ctx)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/cli/command.py", line 208, in cli
    clear=state.clear,
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/core.py", line 574, in ensure_project
    pypi_mirror=pypi_mirror,
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/core.py", line 494, in ensure_virtualenv
    python = ensure_python(three=three, python=python)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/core.py", line 397, in ensure_python
    path_to_python = find_a_system_python(python)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/core.py", line 360, in find_a_system_python
    python_entry = finder.find_python_version(line)
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 113, in find_python_version
    return self.system_path.find_python_version(
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 54, in system_path
    ignore_unsupported=self.ignore_unsupported,
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 451, in create
    ignore_unsupported=ignore_unsupported,
  File "<attrs generated init 75e45f144e3d5510d54dd5fca6730b98fa0220a9>", line 38, in __init__
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 116, in __attrs_post_init__
    self._setup_pyenv()
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/path.py", line 196, in _setup_pyenv
    version_glob_path="versions/*", ignore_unsupported=self.ignore_unsupported
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/python.py", line 156, in create
    sort_function=sort_function, version_glob_path=version_glob_path)
  File "<attrs generated init 3978658e9b85ca2efa769bf31500e76f099ee322>", line 17, in __init__
  File "/usr/local/Cellar/pipenv/2018.11.26_2/libexec/lib/python3.7/site-packages/pipenv/vendor/pythonfinder/models/python.py", line 114, in get_versions
    entry = next(iter(version_path.find_all_python_versions()), None)
AttributeError: 'NoneType' object has no attribute 'find_all_python_versions'
```

## 対応
正直読んでもよくわからなかったです。上半分の`.DS_store`が違うのはいいとしても、`python_versions`が見当たらないのはどいういうわけか...
とおもって、対策を探していたところ、https://github.com/pypa/pipenv/issues/3439 を見つけました。
どうやら、`$HOME/.pyenv/versions`の下に、`.DS_store`があるためだそうです。

## 結論
`rm $HOME/.pyenv/versions/.DS_store`としたら動きました。エラーメッセージはちゃんと読みましょう。