pipenv: 2018.11.14 pipenv commands fail with pyenv

Issue description

We use pyenv to support multiple Python versions on one machine. When using any command with pipenv (version 2018.11.14) it fails with the below traceback.

Expected result

pipenv should execute the commands without errors.

Actual result

$ pipenv install
Traceback (most recent call last):
  File "/usr/local/bin/pipenv", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/cli/command.py", line 249, in install
    editable_packages=state.installstate.editables,
  File "/usr/local/lib/python3.5/dist-packages/pipenv/core.py", line 1725, in do_install
    pypi_mirror=pypi_mirror,
  File "/usr/local/lib/python3.5/dist-packages/pipenv/core.py", line 569, in ensure_project
    pypi_mirror=pypi_mirror,
  File "/usr/local/lib/python3.5/dist-packages/pipenv/core.py", line 489, in ensure_virtualenv
    python = ensure_python(three=three, python=python)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/core.py", line 397, in ensure_python
    path_to_python = find_a_system_python(python)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/core.py", line 360, in find_a_system_python
    python_entry = finder.find_python_version(line)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 113, in find_python_version
    return self.system_path.find_python_version(
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 54, in system_path
    ignore_unsupported=self.ignore_unsupported,
  File "/usr/local/lib/python3.5/dist-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/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/path.py", line 116, in __attrs_post_init__
    self._setup_pyenv()
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/path.py", line 196, in _setup_pyenv
    version_glob_path="versions/*", ignore_unsupported=self.ignore_unsupported
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/python.py", line 154, in create
    sort_function=sort_function, version_glob_path=version_glob_path)
  File "<attrs generated init 66db6832296eebaad0ce8186c2c66d03343025d4>", line 17, in __init__
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/python.py", line 98, in get_versions
    for p in self.get_version_order():
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/python.py", line 70, in get_version_order
    version_order = [versions[v] for v in parse_pyenv_version_order()]
TypeError: 'NoneType' object is not iterable

Steps to replicate

Install pyenv as described in the README on https://github.com/pyenv/pyenv. Install a Python version of your choice using pyenv, e.g. pyenv install 3.7.0. When in your project create a Pipfile and add/change the following section:

[requires]
python_version = "3.7.0"

Now run any pipenv command in your project, .e.g pipenv install or pipenv shell.

---

<details><summary>$ pipenv --support</summary>

Pipenv version: `'2018.11.14'`

Pipenv location: `'/usr/local/lib/python3.5/dist-packages/pipenv'`

Python location: `'/usr/bin/python3'`

Python installations found:

Traceback (most recent call last):
  File "/usr/local/bin/pipenv", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 1114, in invoke
    return Command.invoke(self, ctx)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/pipenv/cli/command.py", line 142, in cli
    get_pipenv_diagnostics()
  File "/usr/local/lib/python3.5/dist-packages/pipenv/help.py", line 33, in get_pipenv_diagnostics
    python_paths = finder.find_all_python_versions()
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 122, in find_all_python_versions
    python_version_dict = getattr(self.system_path, "python_version_dict")
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/pythonfinder.py", line 54, in system_path
    ignore_unsupported=self.ignore_unsupported,
  File "/usr/local/lib/python3.5/dist-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/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/path.py", line 116, in __attrs_post_init__
    self._setup_pyenv()
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/path.py", line 196, in _setup_pyenv
    version_glob_path="versions/*", ignore_unsupported=self.ignore_unsupported
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/python.py", line 154, in create
    sort_function=sort_function, version_glob_path=version_glob_path)
  File "<attrs generated init e41401380504971a8c7664b563d716c9ae61830a>", line 17, in __init__
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/python.py", line 98, in get_versions
    for p in self.get_version_order():
  File "/usr/local/lib/python3.5/dist-packages/pipenv/vendor/pythonfinder/models/python.py", line 70, in get_version_order
    version_order = [versions[v] for v in parse_pyenv_version_order()]
TypeError: 'NoneType' object is not iterable

Thanks for looking into it.