title: pip exists, says no module named pip
body: <p>I have <code>pip</code> installed and can clearly see the file (by using <code>which pip</code>) but, when I go to actually run a command like <code>pip install -r requirements.txt</code> (or even just <code>pip</code> to get help), I get the following error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/local/bin/pip3", line 7, in &lt;module&gt;
      from pip._internal import main
  ModuleNotFoundError: No module named 'pip'
  </code></pre>
  
  <p>I get this whether I'm using pip 2 or 3. What is going on here? Any way to fix it without reinstalling the whole thing?</p>