title: pip is not working when called on a specific Python version
body: <p>Example:</p>
  <pre><code>$ sudo python3.7 -m pip install pandas
  [sudo] password for janis: 
  
  Exception:
  Traceback (most recent call last):
    File &quot;/usr/lib/python3/dist-packages/pip/basecommand.py&quot;, line 215, in main
      status = self.run(options, args)
    File &quot;/usr/lib/python3/dist-packages/pip/commands/install.py&quot;, line 290, in run
      with self._build_session(options) as session:
    File &quot;/usr/lib/python3/dist-packages/pip/basecommand.py&quot;, line 69, in _build_session
      if options.cache_dir else None
    File &quot;/usr/lib/python3.7/posixpath.py&quot;, line 80, in join
      a = os.fspath(a)
  TypeError: expected str, bytes or os.PathLike object, not int
  
  </code></pre>
  <p><em>Ubuntu 18.04</em></p>
  <p>I have already reinstalled python3.7 but it still does not work.</p>
  <p>Any ideas?
  Thanks!</p>