pipenv install/graph fail with 2018.10.9: ImportError: No module named weakref

[greyli]

Issue description

With pipenv 2018.10.9, all the pipenv commands fail (including pipenv --support):

Traceback (most recent call last):
  File "c:\python27\lib\runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "c:\python27\lib\runpy.py", line 72, in _run_code
    exec code in run_globals
  File "C:\Python27\Scripts\pipenv.exe\__main__.py", line 5, in <module>
  File "c:\python27\lib\site-packages\pipenv\__init__.py", line 23, in <module>
    from .cli import cli
  File "c:\python27\lib\site-packages\pipenv\cli\__init__.py", line 3, in <module>
    from .command import cli
  File "c:\python27\lib\site-packages\pipenv\cli\command.py", line 18, in <module>
    from .. import environments
  File "c:\python27\lib\site-packages\pipenv\environments.py", line 4, in <module>
    from .vendor.vistir.misc import fs_str
  File "c:\python27\lib\site-packages\pipenv\vendor\vistir\__init__.py", line 4, in <module>
    from .compat import NamedTemporaryFile, TemporaryDirectory, partialmethod
  File "c:\python27\lib\site-packages\pipenv\vendor\vistir\compat.py", line 34, in <module>
    from .backports.tempfile import NamedTemporaryFile
  File "c:\python27\lib\site-packages\pipenv\vendor\vistir\backports\__init__.py", line 5, in <module>
    from .tempfile import NamedTemporaryFile
  File "c:\python27\lib\site-packages\pipenv\vendor\vistir\backports\tempfile.py", line 16, in <module>
    from backports.weakref import finalize
ImportError: No module named weakref

Enviroment

• pip version: 10.0.1 and 18.1
• Python version: 2.7
• OS: Windows

[immerrr]

FTR, this means backports.weakref module should be added to Py2 dependencies.

[svenpanne]

This looks quite similar to #2950. @immerrr: What exactly should be added where?

[immerrr]

@svenpanne to fix this you can run pip install backports.weakref manually, or wait until #2962 is cleaned up and accepted.