title: syntax error after installing futures package in my virtualenv
body: <p>I'm using virtualenvwrapper. Under my <code>virtualenv</code> with python3, I installed <code>futures</code> successfully, then if I install anything else, it will pop up syntax errors which comes from <code>futures</code> package
  errors are as follows:</p>
  
  <pre class="lang-bash prettyprint-override"><code>$ pip install futures
  Collecting futures
  Installing collected packages: futures
  Successfully installed futures-3.1.1
  $ pip install sklearn
  Traceback (most recent call last):
    File "/data/analytics/yliu/.virtualenvs/alice1/bin/pip", line 7, in &lt;module&gt;
      from pip._internal import main
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/__init__.py", line 40, in &lt;module&gt;
      from pip._internal.cli.autocompletion import autocomplete
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/cli/autocompletion.py", line 8, in &lt;module&gt;
      from pip._internal.cli.main_parser import create_main_parser
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/cli/main_parser.py", line 12, in &lt;module&gt;
      from pip._internal.commands import (
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/commands/__init__.py", line 6, in &lt;module&gt;
      from pip._internal.commands.completion import CompletionCommand
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/commands/completion.py", line 6, in &lt;module&gt;
      from pip._internal.cli.base_command import Command
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/cli/base_command.py", line 24, in &lt;module&gt;
      from pip._internal.index import PackageFinder
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/index.py", line 41, in &lt;module&gt;
      from pip._internal.wheel import Wheel
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/pip/_internal/wheel.py", line 7, in &lt;module&gt;
      import compileall
    File "/usr/local/anaconda3/anaconda/lib/python3.6/compileall.py", line 20, in &lt;module&gt;
      from concurrent.futures import ProcessPoolExecutor
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/concurrent/futures/__init__.py", line 8, in &lt;module&gt;
      from concurrent.futures._base import (FIRST_COMPLETED,
    File "/data/analytics/yliu/.virtualenvs/alice1/lib/python3.6/site-packages/concurrent/futures/_base.py", line 381
      raise exception_type, self._exception, self._traceback
                          ^
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>I checked the answer in this question: <a href="https://stackoverflow.com/questions/44239805/pip-install-error-within-python-3-virtualenv">pip install error within Python 3 virtualenv</a>, but it doesn't work for me. The <code>futures</code> package is indeed installed in the right version of python which is python 3.6.5.</p>
  
  <p>I tried to install futures on python3.6 globally and everything works fine. so it seems something with the <code>futures</code> and the <code>virtualenv</code>. </p>
  
  <p>Can anyone please help me solve this issue?</p>
  
  <p>Thanks so much</p>