title: pip3 --version ImportError
body: <p>pip3 --version is throwing an error as of late and I'm having trouble using pip to install packages to my virtual environment.  This is a new issue but I think it could be due to having too many python versions installed on my computer.</p>
  
  <p>Has anyone else seen this error thrown?  I haven't ever had an error from importlib.util thrown previously.  In addition, this error only creeped up lately.  To my knowledge I haven't done anything to change importlib.util.</p>
  
  <pre><code>Error processing line 1 of /usr/local/Cellar/python3/3.6.2/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/matplotlib-2.0.2-py3.6-nspkg.pth:
  
    Traceback (most recent call last):
      File "/usr/local/Cellar/python3/3.6.2/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site.py", line 168, in addpackage
        exec(line)
      File "&lt;string&gt;", line 1, in &lt;module&gt;
    AttributeError: module 'importlib.util' has no attribute 'module_from_spec'
  
  Remainder of file ignored
  Traceback (most recent call last):
    File "/usr/local/bin/pip3", line 11, in &lt;module&gt;
      load_entry_point('pip==9.0.1', 'console_scripts', 'pip3')()
    File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 561, in load_entry_point
      return get_distribution(dist).load_entry_point(group, name)
    File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2631, in load_entry_point
      return ep.load()
    File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2291, in load
      return self.resolve()
    File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2297, in resolve
      module = __import__(self.module_name, fromlist=['__name__'], level=0)
    File "/usr/local/lib/python3.6/site-packages/pip/__init__.py", line 26, in &lt;module&gt;
      from pip.utils import get_installed_distributions, get_prog
    File "/usr/local/lib/python3.6/site-packages/pip/utils/__init__.py", line 22, in &lt;module&gt;
      from pip.compat import console_to_str, expanduser, stdlib_pkgs
    File "/usr/local/lib/python3.6/site-packages/pip/compat/__init__.py", line 60, in &lt;module&gt;
      from importlib.util import cache_from_source
  ImportError: cannot import name 'cache_from_source'
  </code></pre>