title: ImportError: cannot import name _remove_dead_weakref in django virtualenv
body: <p>After upgrading my Ubuntu desktop to 18.04 from 16.04, the django's virtualenv refuses to start:</p>
  
  <pre><code>(.djangoenv) mw@desktop:~/theapp$ python manage.py runserver
  Traceback (most recent call last):
    File "manage.py", line 8, in &lt;module&gt;
      from django.core.management import execute_from_command_line
    File "/home/mw/.djangoenv/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 10, in &lt;module&gt;
      from django.apps import apps
    File "/home/mw/.djangoenv/local/lib/python2.7/site-packages/django/apps/__init__.py", line 1, in &lt;module&gt;
      from .config import AppConfig   # NOQA
    File "/home/mw/.djangoenv/local/lib/python2.7/site-packages/django/apps/config.py", line 4, in &lt;module&gt;
      from django.core.exceptions import AppRegistryNotReady, ImproperlyConfigured
    File "/home/mw/.djangoenv/local/lib/python2.7/site-packages/django/core/exceptions.py", line 5, in &lt;module&gt;
      from django.utils.encoding import force_text
    File "/home/mw/.djangoenv/local/lib/python2.7/site-packages/django/utils/encoding.py", line 10, in &lt;module&gt;
      from django.utils.functional import Promise
    File "/home/mw/.djangoenv/local/lib/python2.7/site-packages/django/utils/functional.py", line 1, in &lt;module&gt;
      import copy
    File "/usr/lib/python2.7/copy.py", line 52, in &lt;module&gt;
      import weakref
    File "/usr/lib/python2.7/weakref.py", line 14, in &lt;module&gt;
      from _weakref import (
  ImportError: cannot import name _remove_dead_weakref 
  </code></pre>
  
  <p>On the <code>djangoenv</code> and outside it </p>
  
  <pre><code>$ python --version : 
  `Python 2.7.12` 
  </code></pre>
  
  <p>Hoping to resolve the issue, I also install <code>Python 2.7.16</code> on the desktop, so I get:</p>
  
  <pre><code>$ python2.7 --version
  Python 2.7.16
  </code></pre>
  
  <p>Also when I try to install new virtualenv, I get the same error:</p>
  
  <pre><code>$ virtualenv .blaenv
  Running virtualenv with interpreter /home/mw/.djangoenv/bin/python2
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/virtualenv.py", line 21, in &lt;module&gt;
      import logging
    File "/usr/lib/python2.7/logging/__init__.py", line 26, in &lt;module&gt;
      import sys, os, time, cStringIO, traceback, warnings, weakref, collections
    File "/usr/lib/python2.7/weakref.py", line 14, in &lt;module&gt;
      from _weakref import (
  ImportError: cannot import name _remove_dead_weakref
  </code></pre>
  
  <p>Answers to the same error in other contexts did not help and my search led to no more clues.
  So really appreciate your hints.</p>